Add no-non-standard-flag rule

[RunDevelopment]

This implements this comment.

Since we now have 3 rules that report/replace flags, I moved the common flags logic into RegExpContext.

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There is still one problem with this PR: the website:

We probably have to use @typescript-eslint/parser on the website as well.
@ota-meshi Should we do this? If yes, how?

[ota-meshi]

We probably have to use @typescript-eslint/parser on the website as well.
@ota-meshi Should we do this? If yes, how?

Hmm.. @typescript-eslint/parser requires a very large resource because it requires typescript. So I don't want to include @typescript-eslint/parser 😅.
Can you avoid it with example without demo and example with demo using new RegExp()?

[RunDevelopment]

demo using new RegExp()?

Good idea!

[ota-meshi]

LGTM! Thank you!