Use pointers to allow the AST to be modified. Implement visitor support.

[matthewmueller]

This PR:

• Adjusts the AST to use pointers, so it can be modified in place
• Implements a Visitor interface that allows the AST to be traversed and rewritten

The goal of this PR is to allow the community to create compile-time plugins for Templ. Some examples could include:

• Scoped CSS in proposal: deprecate CSS components in favour of 3rd party CSS pre-processors #740
• HTML minification

We could also generalize a few patterns:

• Generating Go code could be a visitor
• Formatting with imports could be another visitor

This PR ended up a bit larger than I would have liked. While you could implement the Visitor pattern without making the AST pointers, it effectively makes it so the AST is read-only. Being able to modify the AST opens up a lot more possibilities.

Let me know if this is a direction you're interested in heading. Tests pass, but there's a few more tests I'd like to contribute before merging.

Also in the #templ channel in the Gophers Slack if you'd like to chat further first.