Refactor Spacing into DynamicSpacing using proc macro

[iamnbutler]

Density tracking issue: #18078

Post-Merge Note: It looks like the enum variant docs aren't showing up correctly. Will work on a fix for this.

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:

.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