Instead of returning a usize for the window id, I'm instead returning a
`WindowHandle<V: View>` where `V` is the type of the window's root view.
@as-cii helped me with a cool technique using generic associated types
where methods on `WindowHandle` can return either T or Option<T>
depending on the `BorrowWindowContext::Result` associated type.
Some example usage...
```rs
let window = cx.add_window(|cx| MyView::new(cx));
let my_view = window.root(cx); // If cx is TestAppContext, returns MyView. Otherwise returns Option<MyView>, because the window could be closed.
```
This isn't insanely beneficial on its own, but I think it will help
clean up our testing story. I'm planning on making `window` more useful
in tests for laying out elements, etc.
- [x] Rework tests that call `add_window` 😅 to expect only a window in
return.
- [x] Get tests passing
- [x] 🚬 test
This adds rudimentary language support for the Nix expression language,
through tree-sitter-nix.
I spent a little bit of time trying to add support for one of the Nix
language servers too, but wasn't able to get any of them running
reliably without crashing, and so I've opted to stick to just the
tree-sitter grammar for now.

View handles are window specific but this global will be doing things
in all windows, that would cause a panic when it attempted to update
a status bar mode indicator in a background window
Co-Authored-By: Mikayla Maki <mikayla@zed.dev>
One of the problems we had is that the status_bar shows a gap between
items, and we want to not add an additional gap for an invisible status
indicator.
This PR adds the next most requested editor feature.
TODO:
- [x] Figure out styles and icons for supported file types with
fixes https://github.com/zed-industries/community/issues/206
Release Notes:
- Added file icons