Opt into html language features

[Whoops]

This opts into VSCode's built-in HTML language features, allowing for autocomplete and documentation based on HTML tags, attributes, etc.

Addresses #20 and #18.

The extension.js implementation is cribbed directly from Jinja's implementation to address https://github.com/microsoft/vscode/issues/160585 as mentioned in the comment.

[SteffenDE]

@Whoops @RedCMD this only works for .heex files, right? I just tried this in a ~H""" block, but there the features don't seem to be available.