Initialize the `SystemAppearance` using the app's global window appea…

…rance (zed-industries#7508)

This PR changes our approach to initializing the `SystemAppearance` so
that we can do it earlier in the startup process.

Previously we were using the appearance from the window, meaning that we
couldn't initialize the value until we first opened the window.

Now we read the `window_appearance` from the `AppContext`. On macOS this
is backed by the
[`effectiveAppearance`](https://developer.apple.com/documentation/appkit/nsapplication/2967171-effectiveappearance)
on the `NSApplication`.

We currently still watch for changes to the appearance at the window
level, as the only hook I could find in the documentation is
[`viewDidChangeEffectiveAppearance`](https://developer.apple.com/documentation/appkit/nsview/2977088-viewdidchangeeffectiveappearance),
which is at the `NSView` level.

In my testing this makes it so Zed appropriately chooses the correct
light/dark theme on startup.

Release Notes:

- N/A

zed 0.121.4

zed 0.121.3

Use `Mutex` instead of a `RefCell` to acquire/release instance buffers (

zed-industries#7291)

This fixes a panic happening when releasing an instance buffer.
Releasing the buffer happens on a different thread but the borrow
checker was not catching it because the metal buffer completion handler
API doesn't have a `Send` marker on it.

Release Notes:

- N/A

zed 0.120.6

zed 0.121.1

collab 0.44.0

Show REVISION too

just kidding (zed-industries#7241)

Release Notes:

- N/A

v0.121.x preview