Add the html_block rule to the markdown renderer

[caleywoods]

The goal for all links within the guides is to evaluate them to see whether or not they leave the ".gov" TLD and if they do, we want to make sure that the class list of the link has usa-link--external applied so that the link gets the external link icon shown when it renders on the page.

Most of the work for link checking is handled by the link_open rendering rule which scans all of the links within the markdown documents and then another portion is handled by html_inline for instances where the HTML is placed inside the markdown document. The research for issue #348 discovered that when an HTML page is included using the 11ty include syntax that neither link_open nor html_inline parse links within the included HTML file.

This commit adds a third renderer rule which scans all chunks of HTML to see if it includes any links that leave the ".gov" domain and then attempts to make sure the usa-link--external class is applied.

Changes proposed in this pull request:

• Add html_block rendering rule so that anchor tags discovered within HTML files have the usa-link--external class applied if their top level domain is something other than .gov

security considerations

No security considerations come to mind.

Testing

• The engineering hiring resume review section contains a link to nber.org that uses the text "in one study", without this code the external link icon is not present but when running this branch locally, you should see the external link icon present on that link as well as the "unbias me" link further down within the alert area.

[cantsin]

LGTM -- tested on main, the usa-link--external links do not show up but with this PR they do 🎉

[caleywoods]

I investigated using String.matchAll() which retains the capture groups when using the regex global flag but ultimately decided the code was harder to understand that way.