suggestion: make license checker a lint plugin

[iuioiua]

Currently, the license checker is a script. Perhaps it could be a lint plugin that ensures that there's one copyright line. The appropriate files would have to be ignored according to the current script and the plugin would have to check that the comment is a Deno.lint.LineComment.

One can see how comments work with a lint plugin in the PR that enabled it:
https://github.com/denoland/deno/pull/29189/files#diff-7eb00668c8b82a2759e5a8fc3da1635117a659a4eb3edc47626de2093af432a7