feat: support type annotations in const statements

- Also minor formatting fix in grammar.js (trimmed whitespace) - Const statements can be annotated with a type like so (reference https://typespec.io/docs/language-basics/values/): ```typespec const someNumber: int32 = 0; const value: string | null = null; ``` Tested with: ``` tree-sitter generate tree-sitter test ``` P.S: Thanks for creating this grammar 🙂 Wanted to play around with TypeSpec but noticed no support for it in Zed, so I started working on an [extension](https://github.com/neoncitylights/zed-typespec) for the editor using (a temporary fork of) this grammar.