gpui: Add scale style property

[someone13574]

Implements scale transformations in GPUI. This doesn't support independent scaling in the x and y directions due to that not being supported by GPUI's text shaping. When an element has the scale property set, it changes its scale relative to its parent.

This PR also switches elements to using a relative coordinate system, where the origin is at the upper-left of the element. Events are also in this relative coordinate system, so an absolute_position property has been added to events so that code such as right-click menus can use it.

Existing code in Zed shouldn't need to many changes to adapt to the new coordinate system. Most code should work as is.

Screencast.from.2025-02-05.23-09-44.webm

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TODO:

• Fix layout_as_root (currently causes Zed to appear broken)
• Scale children of other styled containers
• Map positions in FileDropEvent and MouseExitEvent to relative coordinates
• Fix context menus in Zed (replace position with absolute_position, fix deferred)
• Document new & modified functions
• Tailwind methods?

Release Notes:

• N/A