LibJS: Implement String.prototype.charCodeAt

It's broken for strings with characters outside 7-bit ASCII, but
it's broken in the same way as several existing functions (e.g.
charAt()), so that's probably ok for now.

Libraries/LibJS/Tests/builtins/String/String.prototype-generic-functions.js

@@ -1,6 +1,7 @@
 test("basic functionality", () => {
     const genericStringPrototypeFunctions = [
         "charAt",
+        "charCodeAt",
         "repeat",
         "startsWith",
         "indexOf",

Libraries/LibJS/Tests/builtins/String/String.prototype.charCodeAt.js

@@ -0,0 +1,21 @@
+test("basic functionality", () => {
+    expect(String.prototype.charAt).toHaveLength(1);
+
+    var s = "Foobar";
+    expect(typeof s).toBe("string");
+    expect(s).toHaveLength(6);
+
+    expect(s.charCodeAt(0)).toBe(70);
+    expect(s.charCodeAt(1)).toBe(111);
+    expect(s.charCodeAt(2)).toBe(111);
+    expect(s.charCodeAt(3)).toBe(98);
+    expect(s.charCodeAt(4)).toBe(97);
+    expect(s.charCodeAt(5)).toBe(114);
+    expect(s.charCodeAt(6)).toBe(NaN);
+    expect(s.charCodeAt(-1)).toBe(NaN);
+
+    expect(s.charCodeAt()).toBe(70);
+    expect(s.charCodeAt(NaN)).toBe(70);
+    expect(s.charCodeAt("foo")).toBe(70);
+    expect(s.charCodeAt(undefined)).toBe(70);
+});