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_like.rs

@@ -3,7 +3,7 @@ use gpui::{relative, CursorStyle, DefiniteLength, MouseButton};
 use gpui::{transparent_black, AnyElement, AnyView, ClickEvent, Hsla, Rems};
 use smallvec::SmallVec;
 
-use crate::{prelude::*, ElevationIndex, Spacing};
+use crate::{prelude::*, DynamicSpacing, ElevationIndex};
 
 /// A trait for buttons that can be Selected. Enables setting the [`ButtonStyle`] of a button when it is selected.
 pub trait SelectableButton: Selectable {
@@ -491,10 +491,12 @@ impl RenderOnce for ButtonLike {
                 ButtonLikeRounding::Left => this.rounded_l_md(),
                 ButtonLikeRounding::Right => this.rounded_r_md(),
             })
-            .gap(Spacing::Small.rems(cx))
+            .gap(DynamicSpacing::Base04.rems(cx))
             .map(|this| match self.size {
-                ButtonSize::Large => this.px(Spacing::Medium.rems(cx)),
-                ButtonSize::Default | ButtonSize::Compact => this.px(Spacing::Small.rems(cx)),
+                ButtonSize::Large => this.px(DynamicSpacing::Base06.rems(cx)),
+                ButtonSize::Default | ButtonSize::Compact => {
+                    this.px(DynamicSpacing::Base04.rems(cx))
+                }
                 ButtonSize::None => this,
             })
             .bg(style.enabled(self.layer, cx).background)