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/workspace/src/toolbar.rs

@@ -97,9 +97,9 @@ impl Render for Toolbar {
 
         v_flex()
             .group("toolbar")
-            .p(Spacing::Large.rems(cx))
+            .p(DynamicSpacing::Base08.rems(cx))
             .when(has_left_items || has_right_items, |this| {
-                this.gap(Spacing::Large.rems(cx))
+                this.gap(DynamicSpacing::Base08.rems(cx))
             })
             .border_b_1()
             .border_color(cx.theme().colors().border_variant)
@@ -109,7 +109,7 @@ impl Render for Toolbar {
                     h_flex()
                         .min_h(rems_from_px(24.))
                         .justify_between()
-                        .gap(Spacing::Large.rems(cx))
+                        .gap(DynamicSpacing::Base08.rems(cx))
                         .when(has_left_items, |this| {
                             this.child(
                                 h_flex()