feat(go): Add AST

[amckinney]

Re: #4842

This adds the new Go AST library (written in TypeScript), that will be used to generate Go snippets via the dynamic snippet resolver. The package exists alongside the original Go generator in generators/go-v2, just like generators/python-v2.

A few things to note:

1. This introduces a GoFile node, which extends the Writer.
   • The Writer is responsible for managing all Go import statements and aliases (and is accessible by every AST node), whereas the GoFile is both an AstNode and a Writer.
2. Several AST nodes are currently unused (e.g. Enum, Field), but partial implementations are included upfront so that it's easy to fill in the gaps when we rewrite the Go generator in TypeScript.
3. A variety of tests for the new TypeInstantiation AST node are included in the Snippets.test.ts.