Expose attribute details in CompletionContext

Hopefully this issue will be rendered irrelevant by the future Treesitter migration :crossed_fingers:

In trying to solve the issue of the TailwindCSS extension providing completions in incorrect contexts, I was unable to definitively solve the problem due to lack of information regarding the current context. At least in a way that didn’t require parsing the entire document myself.

Issue is that the extension should only provide completion options inside CSS class related attributes (class, className, etc.). However due to being unable to determine the current attribute name from the completion selectors context I was only able to partially solve the issue by scanning the current line with the cursor and seeing if the first attribute name starts with the word class.

Unfortunately this completely fails if the cursor is not on a line with the attribute name such as:

className={`h-4 w-4 fill-current transition duration-300 ease-out ${
  contactIsInView && "rotate-0 |CURSOR HERE|"
}`}

Ideally, the CompletionContext selectors would be enriched with information from the grammar provider to allow more nuance for extensions.