add requestHandler fall-through - closes #1295

[Rich-Harris]

Changes

This closes #1295, by allowing requestHandler to fall through to other handlers:

snowpackServer.requestHandler(req, res, () => {
  // requested URL didn't correspond to file managed by Snowpack,
  // respond with a custom 404
});

Testing

I couldn't quickly figure out where to add a test for this. Will gladly add a test if given pointers

Docs

requestHandler is currently undocumented

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[FredKSchott]

LGTM! looks like GitHub Actions is down (or at least, giving us strange internal errors) so assuming tests are passing and will confirm locally after merging.