This PR extracts Zig support into an extension and removes the built-in
Zig support from Zed.

There's a small workaround necessary in order for us to set the file
permissions on the `zls` binary so that it can be run. Eventually we'll
want to build this into the extension API, but for now we're just
hard-coding it on the host side.

Release Notes:

- Removed built-in support for Zig, in favor of making it available as
an extension. The Zig extension will be suggested for download when you
open a `.zig` file.