Add config for downloading and installing grammars

[gessen]

Emacs has a variable called treesit-language-source-alist which configures how Emacs downloads and installs tree-sitter grammar. On Emacs master, this list is pre-configured for built-in tree-sitter modes. Could nix-ts-mode provide the same functionality? This is the example from c-ts-mode: https://github.com/emacs-mirror/emacs/blob/master/lisp/progmodes/c-ts-mode.el#L89-L100

[remi-gelinas]

Sure, this seems reasonable! I can take this today.