Properly validate theme(…) function paths in v4

[thecrypticace]

In v4 currently using the theme function in your CSS will show diagnostics about the theme config keys not existing regardless of the theme key.

We've done two things here:

• We compile a candidate with the theme function to see if the theme key exists. While this isn't an optimal solution it works without needing to introduce new APIs into v4 — also prevents us from having to expose the legacy resolved config anywhere in v4.
• We look at all currently registered theme keys to offer suggestions in case you type a theme key incorrectly as long as it's a CSS variable. In v4 suggestions for legacy theme config values is not implemented.