feat: add onRegExpFlags to RegExpValidator.Options and deprecate onFlags

[ota-meshi]

This PR deprecates onFlags in RegExpValidator.Options and adds onRegExpFlags option instead.

The current RegExpValidator.Options onFlags has too many arguments because it has as many arguments as the number of flags. (And we know that more flags will be added in the future. e.g. https://github.com/tc39/proposal-regexp-v-flag)
This has always been a warning to the ESLint we use.

/home/runner/work/regexpp/regexpp/src/parser.ts
Warning:   65:5  warning  Method 'onFlags' has too many parameters (9). Maximum allowed is 8  max-params

/home/runner/work/regexpp/regexpp/src/validator.ts
Warning:   602:5  warning  Method 'onFlags' has too many parameters (9). Maximum allowed is 8  max-params

So instead of giving each flag as multiple arguments, I think it's better to have a single object with the flags information.

However, changing onFlags would be a breaking change, so I think it makes sense to leave onFlags as a deprecated option and introduce a new onRegExpFlags. The name of onRegExpFlags was named with reference to RegularExpressionFlags used in ES specifications.

https://tc39.es/ecma262/#prod-RegularExpressionFlags

[github-actions]

🎉 This PR is included in version 4.3.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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