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/Tests/parser-declaration-in-single-statement-context.js

@@ -0,0 +1,6 @@
+test("Declaration in single-statement context is a syntax error", () => {
+    expect("if (0) const foo = 1").not.toEval();
+    expect("while (0) function foo() {}").not.toEval();
+    expect("for (var 0;;) class foo() {}").not.toEval();
+    expect("do let foo = 1 while (0)").not.toEval();
+});