Move from .lua file extensions to .luau

[autonordev]

I can't imagine any potential breakage here, unless you're modifying the source (which we don't support).

GitHub now supports Luau, so nothing needs to be added to .gitattributes, and this also means that GitHub's syntax highlighting will show Luau features (whereas the current Luau highlighting doesn't).

On VSCode: older Lua extensions, including ones built for Roblox, will not understand .luau extensions, but this shouldn't be an issue for anyone using luau-lsp, and I assume roblox-lsp supports them.

This is not open to contributors, except for the maintainers. A pull request resolving this is not desired; it would cause a lot of noise on the commit history because of how GitHub handles pull requests.

[autonordev]

For another project, I wrote this powershell to recursively do precisely this:

Get-ChildItem "*.lua" -Recurse | Rename-Item -NewName { $_.Name + 'u' }