feat: implement format-on-save in vscode and baml-cli fmt

[sxlijin]

This wires up Dylan's work on setting up a BAML formatter and makes it possible to auto-format some BAML code:

• it is now possible to format-on-save BAML code using the vscode extension by running baml.setDefaultFormatter as a command in vscode
• baml-cli fmt will now work, if run in a dir containing a baml_src dir; it also supports dry-run (will write to stdout instead) and explicitly specifying baml files

The formatter itself is... not quite there yet. So baml-cli fmt will stay a hidden command.

• there are some syntax rules it barfs on (enums? literals? something in integ-tests)
• it rewrites comments possibly too aggressively (e.g. across multiple lines they're concatenated)
• unclear how useful/useless it is without the ability to format template string contents
• our current grammar can be pretty brittle in some cases (e.g. newlines are not allowed in angle brackets, c.f."map<\nstring, string>" is invalid)

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Name	Status	Preview	Comments	Updated (UTC)
baml	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 14, 2024 1:58am

[ellipsis-dev]

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