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/outline_panel/src/outline_panel.rs

@@ -42,7 +42,7 @@ use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};
 use smol::channel;
 use theme::{SyntaxTheme, ThemeSettings};
-use ui::{IndentGuideColors, IndentGuideLayout};
+use ui::{DynamicSpacing, IndentGuideColors, IndentGuideLayout};
 use util::{debug_panic, RangeExt, ResultExt, TryFutureExt};
 use workspace::{
     dock::{DockPosition, Panel, PanelEvent},
@@ -51,8 +51,8 @@ use workspace::{
     ui::{
         h_flex, v_flex, ActiveTheme, ButtonCommon, Clickable, Color, ContextMenu, FluentBuilder,
         HighlightedLabel, Icon, IconButton, IconButtonShape, IconName, IconSize, Label,
-        LabelCommon, ListItem, Scrollbar, ScrollbarState, Selectable, Spacing, StyledExt,
-        StyledTypography, Tooltip,
+        LabelCommon, ListItem, Scrollbar, ScrollbarState, Selectable, StyledExt, StyledTypography,
+        Tooltip,
     },
     OpenInTerminal, WeakItemHandle, Workspace,
 };
@@ -3867,7 +3867,7 @@ impl OutlinePanel {
                 })
                 .child(
                     h_flex()
-                        .pt(Spacing::Small.rems(cx))
+                        .pt(DynamicSpacing::Base04.rems(cx))
                         .justify_center()
                         .child({
                             let keystroke = match self.position(cx) {