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 input error.

• Use unified-args to start processor
• Use unified-args's error handler

Fixes: #30156 (closed)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines