LibJS: Start implementing Date :^)

This adds:

- A global Date object (with `length` property and `now` function)
- The Date constructor (no arguments yet)
- The Date prototype (with `get*` functions)

Libraries/LibJS/Tests/Date.now.js

@@ -0,0 +1,15 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    var last = 0;
+    for (var i = 0; i < 100; ++i) {
+        var now = Date.now();
+        assert(!isNaN(now))
+        assert(now > 1580000000000);
+        assert(now >= last);
+        last = now;
+    }
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}