tools: enable Markdown linter's usage information

Prior to this commit, running `node tools/lint-md --help` and
`node tools/lint-md --version` resulted in an error.

* Use `unified-args` to start processor
* Use `unified-args`'s error handler

Fixes: #30156
PR-URL: #30216
Reviewed-By: Rich Trott <[email protected]>

Author: Derek Lewis