This PR mainlines the current state of new GPUI2-based UI from the
`gpui2-ui` branch.

Included in this is a performance improvement to make use of the
`TextLayoutCache` when calling `layout` for `Text` elements.

Release Notes:

- N/A

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Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>