Add extension for the BQN language

[DavidZwitser]

BQN, while relatively niche, is an amazing language, and Zed deserves support for it.
I've implemented all the features Zed has currently. The readme describes the decisions.

[maxdeviant]

Thanks @DavidZwitser!

We'll need to change the extension ID in the extension.toml to be all lowercase: https://github.com/DavidZwitser/zed-bqn/blob/f4fd839564e8b0e37fe7bc99280d431e1f075ea2/extension.toml#L1

- id = "BQN"
+ id = "bqn"

[DavidZwitser]

@maxdeviant my latest commit has a lowercase ID. Any further action needed?

[maxdeviant]

@maxdeviant my latest commit has a lowercase ID. Any further action needed?

You need to update the submodule in this PR to contain that new commit.

[cla-bot]

We require contributors to sign our Contributor License Agreement, and we don't have @DavidZwitser on file. You can sign our CLA at https://zed.dev/cla. Once you've signed, post a comment here that says '@cla-bot check'.

[DavidZwitser]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[DavidZwitser]

@maxdeviant should be fixed now :) (sorry this is my first PR ever)

[maxdeviant]

Thank you!