Pull request preview link enhancements

[jscholes]

Currently, a single "Preview Tests" link is added to the top of the pull request body, linking to the index.html page for the overall repo on a specific branch. This page displays a table of all test plans, and reviewers then have to locate the individual "Index" and "Review" page links for the pattern being reviewed on the branch.

As a potential enhancement, it would be good to link directly to both of the pattern-specific pages directly from the PR initial comment body (as I've been doing in the corresponding issues). There are some questions posed by this suggestion:

1. How should the script executed in the GitHub action determine which test pattern is the relevant one? Is there already some machinery which marks a pattern directory as new or updated in some way to assist with this?
2. Is it ever appropriate for a pull request and branch to include more than one new or updated pattern? For example, when the exact same component has been implemented twice, once with roving tabindex and once with aria-activedescendant, wouldn't it just create noise and duplicated work to file multiple PRs when the majority of test data is the same?
3. If the answer to point #2 is yes, how should this be handled when generating deep preview links?
4. Should there be a link on all individual pattern index and review pages to return to index.html, just in case?
5. What should the behaviour be when a PR is submitted which doesn't relate to a specific pattern, but to some shared infrastructure instead?

[mcking65]

Perhaps we have some syntax in the PR comment that enables the PR author to specify what preview links are needed and their pattern targets?