Bumps the treesitter version to include
https://github.com/tree-sitter/tree-sitter/pull/5851

Also makes some changes to `cx.spawn_dedicated` to make it work on web:
- remove the blocking `recv` on the main thread
- adds a new variant `TaskState::Rendezvous` to allow this code to
synchronously return a task

`TaskState::Rendezvous` is needed because of how
`spawn_dedicated_thread` works:
- the caller provides a callback that produces a non-`Send` `Future` on
the dedicated thread
- the callback is sent to the dedicated thread, executed, and the
resulting `Task` is sent back to the main thread
- this all happens synchronously, so that `spawn_dedicated_thread`
returns a synchronous `Task`

However, the blocking `recv` on the main thread traps on the main worker
in the browser.

This PR replaces that with a call to `recv_async`, but this requires a
new `Rendezvous` variant on `TaskState` which represents "a task that it
still waiting to receive the underlying handle from the dedicated
thread".

It also enables the `flume/spin` feature on the web, which is needed to
avoid a synchronous lock acquisition in `send` (it's replaced with a
spinlock). Note that `send_async` wouldn't work here because it acquires
the same lock, and it's unnecessary because `send` only blocks when the
channel is full, but it's an unbounded channel.

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Release Notes:

- N/A or Added/Fixed/Improved ...