Add support for 'luau' file extension

[NinjaMandalorian]

Adds support for the .luau file extension, a filetype used most commonly around the Roblox ecosystem, as well as some standalone runtimes.

Hooks created for pre-commit (like StyLua's from 5 years ago) are aimed at the "lua" type. Adding this allows for backwards compatibility.

[asottile]

hmm it seems to me that luau is a superset of lua so I'm not sure it makes sense to tag it as lua (since a luau file may use features not expected by a tool targetting lua)

[NinjaMandalorian]

hmm it seems to me that luau is a superset of lua so I'm not sure it makes sense to tag it as lua (since a luau file may use features not expected by a tool targetting lua)

From my understanding, the deviation is more around adding typing, as luau is fully compatible with Lua 5.1, and does inherit features from future versions, albeit not fully.

I'll edit the PR either way to remove lua due to these differences.