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/repl/src/notebook/notebook_ui.rs

@@ -273,7 +273,7 @@ impl NotebookEditor {
 
     fn button_group(cx: &ViewContext<Self>) -> Div {
         v_flex()
-            .gap(Spacing::Small.rems(cx))
+            .gap(DynamicSpacing::Base04.rems(cx))
             .items_center()
             .w(px(CONTROL_SIZE + 4.0))
             .overflow_hidden()
@@ -299,14 +299,14 @@ impl NotebookEditor {
         v_flex()
             .max_w(px(CONTROL_SIZE + 4.0))
             .items_center()
-            .gap(Spacing::XXLarge.rems(cx))
+            .gap(DynamicSpacing::Base16.rems(cx))
             .justify_between()
             .flex_none()
             .h_full()
-            .py(Spacing::XLarge.px(cx))
+            .py(DynamicSpacing::Base12.px(cx))
             .child(
                 v_flex()
-                    .gap(Spacing::Large.rems(cx))
+                    .gap(DynamicSpacing::Base08.rems(cx))
                     .child(
                         Self::button_group(cx)
                             .child(
@@ -390,7 +390,7 @@ impl NotebookEditor {
             )
             .child(
                 v_flex()
-                    .gap(Spacing::Large.rems(cx))
+                    .gap(DynamicSpacing::Base08.rems(cx))
                     .items_center()
                     .child(Self::render_notebook_control(
                         "more-menu",
@@ -468,8 +468,8 @@ impl Render for NotebookEditor {
             .items_start()
             .size_full()
             .overflow_hidden()
-            .px(Spacing::XLarge.px(cx))
-            .gap(Spacing::XLarge.px(cx))
+            .px(DynamicSpacing::Base12.px(cx))
+            .gap(DynamicSpacing::Base12.px(cx))
             .bg(cx.theme().colors().tab_bar_background)
             .child(
                 v_flex()