LibJS: Ensure function declarations don't leak outside function scopes

When using VM::set_variable() to put the created ScriptFunction onto a
ScopeObject, we would previously unexpectedly reach the global object as
set_variable() checks each traversed scope for an existing Variable with
the given name - which would cause a leak of the inner function past the
outer function (we even had a test expecting that behaviour!). Now we
first declare functions (as DeclarationKind::Var) before setting them.
This will need some more work to make hoisting across non-lexical scopes
work, but it fixes this specific issue for now.

Fixes #6766.

Userland/Libraries/LibJS/Tests/functions/function-nesting.js

@@ -0,0 +1,15 @@
+test("issue #6766, nested functions should not leak to global object", () => {
+    function foo() {
+        function bar() {
+            function baz() {
+                return 42;
+            }
+            return baz();
+        }
+        return bar();
+    }
+    expect(foo()).toBe(42);
+    expect(globalThis.foo).toBeUndefined();
+    expect(globalThis.bar).toBeUndefined();
+    expect(globalThis.baz).toBeUndefined();
+});