Support for Group Modifiers in Regular Expressions (regexp)

[hansl]

MDN documentation: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Regular_expressions/Modifier

Part of ES2026. Would fix more than 70 failures on RegExp test262 (test/built-ins/RegExp/regexp-modifiers is about 70 tests and all fail, and more tests for modifiers).

Filed an issue on regress (ridiculousfish/regress#132). Not sure the amount of work required to support this.