This PR adds undocumented functionality for loading custom language
plugins at runtime. I don't intend to expose the functionality to end
users yet, but this will allow the team to test the capability
internally.
### Implementation
There isn't much new code in Zed. Most of the work here is within
Tree-sitter, in PRs https://github.com/tree-sitter/tree-sitter/pull/1864
and https://github.com/tree-sitter/tree-sitter/pull/2840, which allow
Tree-sitter to load languages from WASM blobs. I've tested the
functionality in Tree-sitter's test suite and via its CLI, but having it
wired into Zed allows us to test the functionality more fully.
### Details
Now, on startup, Zed will look for subdirectories inside of
`~/Application Support/plugins`. These subdirectories are expected to
look similar to the per-language subdirectories in
[`crates/zed2/src/languages`](https://github.com/zed-industries/zed/tree/main/crates/zed2/src/languages),
except that they also contain a `.wasm` file for the parser itself.
I'll add more details here as I go.
