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/assistant/src/assistant_panel.rs

@@ -447,7 +447,7 @@ impl AssistantPanel {
                     );
                 let _pane = cx.view().clone();
                 let right_children = h_flex()
-                    .gap(Spacing::XSmall.rems(cx))
+                    .gap(DynamicSpacing::Base02.rems(cx))
                     .child(
                         IconButton::new("new-chat", IconName::Plus)
                             .on_click(
@@ -4838,7 +4838,7 @@ impl ConfigurationView {
             )
             .child(
                 div()
-                    .p(Spacing::Large.rems(cx))
+                    .p(DynamicSpacing::Base08.rems(cx))
                     .bg(cx.theme().colors().surface_background)
                     .border_1()
                     .border_color(cx.theme().colors().border_variant)
@@ -4872,7 +4872,7 @@ impl Render for ConfigurationView {
             .overflow_y_scroll()
             .child(
                 v_flex()
-                    .p(Spacing::XXLarge.rems(cx))
+                    .p(DynamicSpacing::Base16.rems(cx))
                     .border_b_1()
                     .border_color(cx.theme().colors().border)
                     .gap_1()
@@ -4886,7 +4886,7 @@ impl Render for ConfigurationView {
             )
             .child(
                 v_flex()
-                    .p(Spacing::XXLarge.rems(cx))
+                    .p(DynamicSpacing::Base16.rems(cx))
                     .mt_1()
                     .gap_6()
                     .flex_1()