Implement `implements` detection

[bryceknz]

Currently this action only supports the following keywords (and their derivatives):

• 'close'
• 'fix'
• 'resolve'

It would be useful if it also detected 'implements'.

[simoneb]

These keywords are not arbitrary, and it might be worth documenting that with a comment perhaps.

https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/using-keywords-in-issues-and-pull-requests

I presume we use these keywords because they are the same that GitHub understands in order to link a PR to an issue.