Added .assert_shared_state() to NeovimBackedTestContext – although it's
not strictly necessary to show the expected behaviour in the test file
(as we can just compare to neovim's JSON recording), it makes it much
easier to understand what we're testing.

Fixes: zed-industries/community#470

This PR adds a new mouse event type for catching when a click happens
outside of a given region.

This was added because I noticed a 'race condition' between the context
menu and the buttons which deploy a context menu. Buttons use on
an`on_click()` handler to deploy the context menu, but the context menu
was closing itself with an `on_down_out()` handler. This meant that the
order of operations was:

0. Context menu is open
1. User presses down on the button, _outside of the context menu_ 
2. `on_down_out()` is fired, closing the context menu
3. User releases the mouse
4. `click()` is fired, checks the state of the context menu, finds that
it's closed, and so opens it

You can see this behavior demonstrated with this video with a long-click
here:


https://github.com/zed-industries/zed/assets/2280405/588234c3-1567-477f-9a12-9e6a70643527

~~Switching from `on_down_out()` to `on_click_out()` means that the
click handler for the button can close the menu before the context menu
gets a chance to close itself.~~

~~However, GPUI does not have an `on_click_out()` event, hence this
PR.~~

~~Here's an example of the new behavior, with the same long-click
action:~~


https://github.com/zed-industries/zed/assets/2280405/a59f4d6f-db24-403f-a281-2c1148499413

Unfortunately, this `click_out` is the incorrect event for this to
happen on. This PR now adds a mechanism for delaying the firing of a
cancel action so that toggle buttons can signal that this on_down event
should not result in a menu closure.

Release Notes:

* Made context menus deployed from buttons toggle, instead of
hide-and-re-show, visibility on click

Add 'delay_cancel()' method and on_down handler to relevant buttons

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

Co-Authored-By: Antonio Scandurra <antonio@zed.dev>

Co-Authored-By: Antonio Scandurra <antonio@zed.dev>

Co-Authored-By: Antonio Scandurra <antonio@zed.dev>

Co-Authored-By: Antonio Scandurra <antonio@zed.dev>

zzz