WPT harness: parse meta comments, raise error if config provided for nonexistent test

[npaun]

• Parse WPT meta comments found at the beginning of tests, in the form of "// META key=value"
• Implement the "script" directive which includes another script
  • Because we're using unsafeEval to run the test code, I had to add a guard to make sure each script is included only once.
  • Also gets rid of our ad-hoc TestOptions.includeFile feature
• Throw an error at the beginning of a test suite if a file listed in config wasn't actually discovered in the WPT repo. This way if you thought we were running a certain test but it wasn't working, there won't be a silent failure.

[npaun]

...

[npaun]

@anonrig Let me know if this makes things better for CLion too...

[npaun]

Damn. Still no windows. At this point the tsconfig is so simple.

[anonrig]

We need to fix Windows before landing this PR but otherwise it looks good to me

[npaun]

The internal build failures aren't related to anything in this PR; merging.