suggestion: convert the doc checker into a custom lint plugin

The doc checker (https://github.com/denoland/std/blob/main/_tools/check_docs.ts) is currently a script. But this can be converted to a lint plugin thanks to comments being supported in canary in https://github.com/denoland/deno/pull/29189. For someone to implement this, they'd need to create a new lint plugin and use [`npm:comment-parser`](https://www.npmjs.com/package/comment-parser) to parse then process comments.