feat: HTML/CSS/JS ContextWriter

[alehechka]

Overview

This is another step toward HTML LSP support in templ, related to the discussion in #498. This PR specifically implements the ContextWriter that @a-h proposed in this comment.

With these changes, I updated nearly all the unit tests to include an expected output format for each context where appropriate (i.e. JS for JS, HTML for HTML). We could add expected output for each individual context, but I skipped on that for now since they would all end up being a huge series of whitespace characters.