LibJS: Distinguish between statement and declaration

This separates matching/parsing of statements and declarations and
fixes a few edge cases where the parser would incorrectly accept a
declaration where only a statement is allowed - for example:

    if (foo) const a = 1;
    for (var bar;;) function b() {}
    while (baz) class c {}

Libraries/LibJS/AST.h

@@ -172,6 +172,12 @@ public:
 class Declaration : public Statement {
 };
 
+class ErrorDeclaration final : public Declaration {
+public:
+    Value execute(Interpreter&, GlobalObject&) const override { return js_undefined(); }
+    const char* class_name() const override { return "ErrorDeclaration"; }
+};
+
 class FunctionNode {
 public:
     struct Parameter {