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/button/button.rs

@@ -1,7 +1,9 @@
 #![allow(missing_docs)]
 use gpui::{AnyView, DefiniteLength};
 
-use crate::{prelude::*, ElevationIndex, IconPosition, KeyBinding, Spacing, TintColor};
+use crate::{
+    prelude::*, Color, DynamicSpacing, ElevationIndex, IconPosition, KeyBinding, TintColor,
+};
 use crate::{
     ButtonCommon, ButtonLike, ButtonSize, ButtonStyle, IconName, IconSize, Label, LineHeightStyle,
 };
@@ -398,7 +400,7 @@ impl RenderOnce for Button {
 
         self.base.child(
             h_flex()
-                .gap(Spacing::Small.rems(cx))
+                .gap(DynamicSpacing::Base04.rems(cx))
                 .when(self.icon_position == Some(IconPosition::Start), |this| {
                     this.children(self.icon.map(|icon| {
                         ButtonIcon::new(icon)
@@ -412,7 +414,7 @@ impl RenderOnce for Button {
                 })
                 .child(
                     h_flex()
-                        .gap(Spacing::Medium.rems(cx))
+                        .gap(DynamicSpacing::Base06.rems(cx))
                         .justify_between()
                         .child(
                             Label::new(label)