LibJS: Improve UpdateExpression::execute()

- Let undefined variables throw a ReferenceError by using
  Identifier::execute() rather than doing variable lookup manually and
  ASSERT()ing
- Coerce value to number rather than ASSERT()ing
- Make code DRY
- Add tests

Libraries/LibJS/Tests/update-expressions-basic.js

@@ -0,0 +1,54 @@
+load("test-common.js");
+
+try {
+    assertThrowsError(() => {
+        ++x;
+    }, {
+        error: ReferenceError,
+        message: "'x' not known"
+    });
+
+    var n = 0;
+    assert(++n === 1);
+    assert(n === 1);
+
+    var n = 0;
+    assert(n++ === 0);
+    assert(n === 1);
+
+    var n = 0;
+    assert(--n === -1);
+    assert(n === -1);
+
+    var n = 0;
+    assert(n-- === 0);
+    assert(n === -1);
+
+    var a = [];
+    assert(a++ === 0);
+    assert(a === 1);
+
+    var b = true;
+    assert(b-- === 1);
+    assert(b === 0);
+
+    var s = "foo";
+    assert(isNaN(++s));
+    assert(isNaN(s));
+
+    var s = "foo";
+    assert(isNaN(s++));
+    assert(isNaN(s));
+
+    var s = "foo";
+    assert(isNaN(--s));
+    assert(isNaN(s));
+
+    var s = "foo";
+    assert(isNaN(s--));
+    assert(isNaN(s));
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}