Support for Emacs 29+ treesitter integration

[sjrmanning]

Now that Emacs 29 has built-in treesitter support and there's a Swift grammar here, it looks like we'd need a swift-ts-mode to actually take advantage of it. I found some notes on creating a treesitter-powered major mode here.

[taku0]

It's very promising. It will improve accuracy, performance, and maintainability. It seems that it requires a lot of work to implement the new mode.