Add ability to configure RegExp flags

[ekzobrain]

Here https://github.com/sagold/json-schema-library/blob/main/src/keywords/pattern.ts#L15 u flag is hardcoded. But nowadays it's better to use v which introduced more advanced Unicode support (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/unicodeSets). But it is also more strict to redudant character escapes as well. So it is actually nessesary to be able to configure which flags to use. It may be a configuration option which defaults to u, or may be (which is better imo) a callback that is used to instantiate a regexp with signature like this: (pattern: string): RegExp
Also it is usually required to catch RegExp compilation errors, so that one invalid pattern does not break the whole schema validation. With a callback option users may implement error catching themselves.

[sagold]

Since you already put thoughts to a solution:
What about adding a schema-property patternFlags to configure the regExp per pattern?

e.g. like this:

const pattern = new RegExp(schema.pattern, schema.patternFlags ?? "v");

[ekzobrain]

I think a special custom keyword may be added as an option (Ajv also supports such one). But they also support a global callback to override default instantiation of a RegExp. As for me, I would go with your suggested solution because it makes v a default flag. I always use it and it is the most recommended now (for example HTMLElement.pattern RegExps are compiled with that flag by automatically). But this would be a minor breaking change for your lib and some uses may face issues with a default v flag if their existing regexp's are not strict enough to conform. If it does not matter much - then it's ok...