This is the initial PR to enable WPT reporting, so we can add our results to wpt.fyi.

Here's what we have so far:

• If GEN_TEST_REPORT=1 is set when running a WPT test, we get a wpt report compatible JSON result.
• If GEN_TEST_STATS=1 is set when running a WPT test, we get a Markdown table row that summarizes the results.
• We already have GEN_TEST_CONFIG=1 that creates a blank template for test configuration, listing each test in a WPT module.

What's missing - I plan to add these in further PRs later

• Handle variants: Each variant declared in a // META comment at the top of a file should appear as a separate test. The test URL then gets ?variant=whatev aqdded as a query param
• Minor detail: set isExpectedFailure correctly in the report. I hardcoded it to true, which is correct in main but might not be in local dev.
• A script to run all these WPT tests under Bazel and gather the output into one report.
  • GEN_TEST_REPORT: Fill out some metadata about workerd version/commit and add all results. Upload to wpt.fyi
  • GEN_TEST_STATS: Append all of these table rows into one table and put it in the action output somewhere so we can get a human readable summary