Support basic inlay hints (#2660)

Part of https://github.com/zed-industries/community/issues/138
Part of https://linear.app/zed-industries/issue/Z-477/inlay-hints

Supports LSP requests for inlay hints, LSP /refresh request to reload
them.
Reworks DisplayMap and underlying layer to unite suggestions with inlay
hints into new, `InlayMap`.
Adds a hint cache inside `Editor` that tracks buffer/project/LSP request
events, updates the hints and ensures opened editors are showing up to
date text hints on top.

Things left to do after this PR:
* docs on how to configure inlay hints
* blogpost
* dynamic hints: resolve, hover, navigation on click, etc.

Release Notes:

- Added basic support of inlay hints