LibJS: Support VariableDeclaration with multiple declarators

This patch adds support in the parser and interpreter for this:

    var a = 1, b = 2, c = a + b;

VariableDeclaration is now a sequence of VariableDeclarators. :^)

Libraries/LibJS/Tests/var-multiple-declarator.js

@@ -0,0 +1,11 @@
+function assert(x) { if (!x) throw 1; }
+
+try {
+    var a = 1, b = 2, c = a + b;
+    assert(a === 1);
+    assert(b === 2);
+    assert(c === 3);
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}