Refactor Spacing into DynamicSpacing using proc macro (#20504)

Density tracking issue: #18078 

This PR refactors our spacing system to use a more flexible and
maintainable approach. We've replaced the static `Spacing` enum with a
dynamically generated `DynamicSpacing` enum using a proc macro.

Enum variants now use a `BaseXX` format, where XX = the pixel value @
default rem size and the default UI density.

For example:

`CustomSpacing::Base16` would return 16px at the default UI scale &
density.

I'd love to find another name other than `Base` that is clear (to avoid
base_10, etc confusion), let me know if you have any ideas!

Changes:

- Introduced a new `derive_dynamic_spacing` proc macro to generate the
`DynamicSpacing` enum
- Updated all usages of `Spacing` to use the new `DynamicSpacing`
- Removed the `custom_spacing` function, mapping previous usages to
appropriate `DynamicSpacing` variants
- Improved documentation and type safety for spacing values

New usage example:

```rust
.child(
    div()
        .flex()
        .flex_none()
        .m(DynamicSpacing::Base04.px(cx))
        .size(DynamicSpacing::Base16.rems(cx))
        .children(icon),
)
```

vs old usage example:

```
.child(
    div()
        .flex()
        .flex_none()
        .m(Spacing::Small.px(cx))
        .size(custom_spacing(px(16.)))
        .children(icon),
)
```

Release Notes:

- N/A

crates/ui/src/components/modal.rs

@@ -1,8 +1,8 @@
 #![allow(missing_docs)]
 
 use crate::{
-    h_flex, v_flex, Clickable, Color, Headline, HeadlineSize, IconButton, IconButtonShape,
-    IconName, Label, LabelCommon, LabelSize, Spacing,
+    h_flex, v_flex, Clickable, Color, DynamicSpacing, Headline, HeadlineSize, IconButton,
+    IconButtonShape, IconName, Label, LabelCommon, LabelSize,
 };
 use gpui::{prelude::FluentBuilder, *};
 use smallvec::SmallVec;
@@ -78,7 +78,7 @@ impl RenderOnce for Modal {
                     .id(self.container_id.clone())
                     .w_full()
                     .flex_1()
-                    .gap(Spacing::Large.rems(cx))
+                    .gap(DynamicSpacing::Base08.rems(cx))
                     .when_some(
                         self.container_scroll_handler,
                         |this, container_scroll_handle| {
@@ -160,10 +160,10 @@ impl RenderOnce for ModalHeader {
             .flex_none()
             .justify_between()
             .w_full()
-            .px(Spacing::XLarge.rems(cx))
-            .pt(Spacing::Large.rems(cx))
-            .pb(Spacing::Small.rems(cx))
-            .gap(Spacing::Large.rems(cx))
+            .px(DynamicSpacing::Base12.rems(cx))
+            .pt(DynamicSpacing::Base08.rems(cx))
+            .pb(DynamicSpacing::Base04.rems(cx))
+            .gap(DynamicSpacing::Base08.rems(cx))
             .when(self.show_back_button, |this| {
                 this.child(
                     IconButton::new("back", IconName::ArrowLeft)
@@ -253,7 +253,7 @@ impl RenderOnce for ModalFooter {
         h_flex()
             .flex_none()
             .w_full()
-            .p(Spacing::Large.rems(cx))
+            .p(DynamicSpacing::Base08.rems(cx))
             .justify_between()
             .child(div().when_some(self.start_slot, |this, start_slot| this.child(start_slot)))
             .child(div().when_some(self.end_slot, |this, end_slot| this.child(end_slot)))
@@ -330,7 +330,7 @@ impl RenderOnce for Section {
         let children = if self.contained {
             v_flex()
                 .flex_1()
-                .when(self.padded, |this| this.px(Spacing::XLarge.rems(cx)))
+                .when(self.padded, |this| this.px(DynamicSpacing::Base12.rems(cx)))
                 .child(
                     v_flex()
                         .w_full()
@@ -338,17 +338,17 @@ impl RenderOnce for Section {
                         .border_1()
                         .border_color(cx.theme().colors().border)
                         .bg(section_bg)
-                        .py(Spacing::Medium.rems(cx))
-                        .gap_y(Spacing::Small.rems(cx))
+                        .py(DynamicSpacing::Base06.rems(cx))
+                        .gap_y(DynamicSpacing::Base04.rems(cx))
                         .child(div().flex().flex_1().size_full().children(self.children)),
                 )
         } else {
             v_flex()
                 .w_full()
                 .flex_1()
-                .gap_y(Spacing::Small.rems(cx))
+                .gap_y(DynamicSpacing::Base04.rems(cx))
                 .when(self.padded, |this| {
-                    this.px(Spacing::Medium.rems(cx) + Spacing::Medium.rems(cx))
+                    this.px(DynamicSpacing::Base06.rems(cx) + DynamicSpacing::Base06.rems(cx))
                 })
                 .children(self.children)
         };
@@ -359,7 +359,7 @@ impl RenderOnce for Section {
             .child(
                 v_flex()
                     .flex_none()
-                    .px(Spacing::XLarge.rems(cx))
+                    .px(DynamicSpacing::Base12.rems(cx))
                     .children(self.header)
                     .when_some(self.meta, |this, meta| {
                         this.child(Label::new(meta).size(LabelSize::Small).color(Color::Muted))
@@ -399,7 +399,7 @@ impl RenderOnce for SectionHeader {
         h_flex()
             .id(self.label.clone())
             .w_full()
-            .px(Spacing::Large.rems(cx))
+            .px(DynamicSpacing::Base08.rems(cx))
             .child(
                 div()
                     .h_7()
@@ -407,7 +407,7 @@ impl RenderOnce for SectionHeader {
                     .items_center()
                     .justify_between()
                     .w_full()
-                    .gap(Spacing::Small.rems(cx))
+                    .gap(DynamicSpacing::Base04.rems(cx))
                     .child(
                         div().flex_1().child(
                             Label::new(self.label.clone())