feat(commands): add regular expression prefixes

[cubedhuang]

About

This adds regex prefixes to the CommandClient and adds one in /test/class.ts. It addresses #306.

Status

• [x] These changes have been tested against Discord API or do not contain API change.
• [ ] This PR includes only documentation changes, no code changes.
• [ ] This PR introduces some Breaking changes.

[cubedhuang]

lgtm, fix the lint issue and i think we can merge this

i think this is an eslint issue because .startsWith should always return a boolean. the issue might be that the eslint version is using an old version of typescript which doesn't have better type guards, not sure how to fix

[Helloyunho]

merging for now, I think I can fix the lint error