LibJS: Make invalid RegExp flags a SyntaxError at parse time

This patch changes the validation of RegExp flags (checking for
invalid and duplicate values) from a SyntaxError at runtime to a
SyntaxError at parse time - it's not something that's supposed to be
catchable.
As a nice side effect, this simplifies the RegExpObject constructor a
bit, as it can no longer throw an exception and doesn't have to validate
the flags itself.

Userland/Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.test.js

@@ -49,10 +49,10 @@ test("flag and options", () => {
     expect(re.unicode).toBe(false);
 
     expect(() => {
-        /foo/gg;
+        Function("/foo/gg");
     }).toThrowWithMessage(SyntaxError, "Repeated RegExp flag 'g'");
 
     expect(() => {
-        /foo/x;
+        Function("/foo/x");
     }).toThrowWithMessage(SyntaxError, "Invalid RegExp flag 'x'");
 });