LibJS: Parse the RegExp.prototype.hasIndices flag

Author: https://github.com/trflynn89
Commit: d1e06b00e3
Pull-request: https://github.com/SerenityOS/serenity/pull/8596
Reviewed-by: https://github.com/linusg

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

@@ -1,5 +1,6 @@
 test("basic functionality", () => {
     expect(/foo/.flags).toBe("");
+    expect(/foo/d.flags).toBe("d");
     expect(/foo/g.flags).toBe("g");
     expect(/foo/i.flags).toBe("i");
     expect(/foo/m.flags).toBe("m");
@@ -7,5 +8,5 @@ test("basic functionality", () => {
     expect(/foo/u.flags).toBe("u");
     expect(/foo/y.flags).toBe("y");
     // prettier-ignore
-    expect(/foo/sgimyu.flags).toBe("gimsuy");
+    expect(/foo/dsgimyu.flags).toBe("dgimsuy");
 });