Add Choose Language Dialog

[maxamundsen]

Adds choose_language command and dialog for selecting the language of the current buffer.

• The language strings in the dialog are generated at compile time from the Buffer.Lang enum. Perhaps we want a 'prettier' string to print, but this is a simple solution that doesn't require manually mapping a string for each enum.
• Once the dialog is open, the current language is pushed to the top, and it will show some text indicating that it is selected.
• If the build window is the active editor, the buffer language will not be changed, and a warning will pop up, alerting the user that you can't change the language of the build panel.
• When selecting Plain_Text, it will clear all of the active regions and tokens for the buffer, clearing all highlighting. (Originally, I implemented a basic tokenizer which set all tokens to .default, later realizing this was totally unnecessary 😂)

[focus-editor]

Looking pretty good. Thanks for your attention to detail. I'm going to try it out and merge if ok

[focus-editor]

Overall I'm impressed with the quality of your PR. Thanks for your work.

I've made a few changes and am planning to make a few more later. This was actually the next item on my list and if you submitted it a few hours later there might've been conflicts. It might be a good idea to check with us before starting a sizeable piece of work to avoid conflicts or possible PR rejection.