semantic pr footer should support linking the same way github does

[Stephen Mathieson]

The semantic-pr-footer action only supports a tiny subset of the keywords GitHub supports. This seems like an unnecessary limitation, which has already caused confusion (https://github.com/dequelabs/axe-api-team/pull/460).

See https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword