Incorrect return type of XRegExp in types/index.d.ts #327

Currently, the return type of XRegExp declared in types/index.d.ts is

declare function XRegExp(pattern: string, flags?: string): RegExp;

However, this annotation is incorrect because it does not capture the two custom properties <regexp>.xregexp.source and <regexp>.xregexp.flags as indicated in the API documentation.

I'm happy to open a PR to fix this issue if necessary.