Try parallel workers and multibar progress

[fulldecent]

Work plan

• Create a worker thread script
  • It accepts a message with one file to process
  • It starts working synchronously (first mock this to just wait randomly 0...2 seconds and respond with a good result or failures)
  • It responds with a message with that file's test report (as formatted text with console color? or as JSON?)
• The main script will use cli-progress with multibar, manage the multithreading, and consolidate the output
• First make this all work on local computer testing runs
• Then check on GitHub Actions, is this working there too without freezing? (solves Update cli-progress to work on GitHub Actions #141) Or do we need to have separate code paths for non-interactive GA runs?

Follow on work

• Report progress back to https://gitlab.com/html-validate/html-validate/-/issues/273

[fulldecent]

Fixed at #144