LibJS: Implement `RegExpPrototype::exec()'

This implements *only* the builtin exec() function.

Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.exec.js

@@ -0,0 +1,58 @@
+test("basic functionality", () => {
+    let re = /foo/;
+    expect(re.exec.length).toBe(1);
+
+    let res = re.exec("foo");
+    expect(res.length).toBe(1);
+    expect(res[0]).toBe("foo");
+    expect(res.groups).toBe(undefined);
+    expect(res.index).toBe(0);
+});
+
+test("basic unnamed captures", () => {
+    let re = /f(o.*)/;
+    let res = re.exec("fooooo");
+
+    expect(res.length).toBe(2);
+    expect(res[0]).toBe("fooooo");
+    expect(res[1]).toBe("ooooo");
+    expect(res.groups).toBe(undefined);
+    expect(res.index).toBe(0);
+});
+
+test("basic named captures", () => {
+    let re = /f(?<os>o.*)/;
+    let res = re.exec("fooooo");
+
+    expect(res.length).toBe(1);
+    expect(res.index).toBe(0);
+    expect(res[0]).toBe("fooooo");
+    expect(res.groups).not.toBe(undefined);
+    expect(res.groups.os).toBe("ooooo");
+});
+
+test("basic index", () => {
+    let re = /foo/;
+    let res = re.exec("abcfoo");
+
+    expect(res.length).toBe(1);
+    expect(res.index).toBe(3);
+    expect(res[0]).toBe("foo");
+});
+
+test("basic index with global and initial offset", () => {
+    let re = /foo/g;
+    re.lastIndex = 2;
+    let res = re.exec("abcfoo");
+
+    expect(res.length).toBe(1);
+    expect(res.index).toBe(3);
+    expect(res[0]).toBe("foo");
+});
+
+test("not matching", () => {
+    let re = /foo/;
+    let res = re.exec("bar");
+
+    expect(res).toBe(null);
+});