fix: rewrite isUnknownOption as stripped down full parse

[shadowspawn]

There have been a lot of issues related to unknown-options-as-args. I had a look at rewriting isUnknownOption based directly on the full parser to see how much code was needed to do "real" parsing. Not too bad. (Alternative PRs coming with lighter weight approach, patching the existing code!)

Open Issues: #312 #359 #376 #501

Closed Issues: #226 #231 #386 #437 #453

Open Pull Requests: #502

Closed Pull Requests #207 #227 #238 #326 #386 #394 #400 #438 #456 #458

[shadowspawn]

I was frustrated by ongoing issues with unknown-options-as-args, and some regexp that seemed misguided, and thought a rewrite might be justified. As I learnt more, I found I could eliminate some regexp and fix the known issues in light-weight way.

Wanted to post the work in case useful to look at again in future.

I think a full rewrite is too much churn for now so I'll close PR.

[shadowspawn]

I liked the hasAnyFlag rewrite which was part of this. I could do that as a separate PR if wanted.