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/icon.rs

@@ -90,10 +90,10 @@ impl IconSize {
     pub fn square_components(&self, cx: &mut WindowContext) -> (Pixels, Pixels) {
         let icon_size = self.rems() * cx.rem_size();
         let padding = match self {
-            IconSize::Indicator => Spacing::None.px(cx),
-            IconSize::XSmall => Spacing::XSmall.px(cx),
-            IconSize::Small => Spacing::XSmall.px(cx),
-            IconSize::Medium => Spacing::XSmall.px(cx),
+            IconSize::Indicator => DynamicSpacing::Base00.px(cx),
+            IconSize::XSmall => DynamicSpacing::Base02.px(cx),
+            IconSize::Small => DynamicSpacing::Base02.px(cx),
+            IconSize::Medium => DynamicSpacing::Base02.px(cx),
         };
 
         (icon_size, padding)