LibJS: Implement RegExp.prototype.compile

This is an Annex B extension to RegExp.prototype.

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

@@ -0,0 +1,17 @@
+test("basic functionality", () => {
+    let re = /foo/;
+    expect(re.compile.length).toBe(2);
+
+    re.compile("bar");
+    expect(re.test("foo")).toBeFalse();
+    expect(re.test("bar")).toBeTrue();
+
+    expect(re.unicode).toBeFalse();
+    re.compile("bar", "u");
+    expect(re.unicode).toBeTrue();
+
+    re.compile(/baz/g);
+    expect(re.global).toBeTrue();
+    expect(re.test("bar")).toBeFalse();
+    expect(re.test("baz")).toBeTrue();
+});