LibJS: Don't hang when parsing invalid destructuring assignment target

Previously, certain crafted input could cause the JS parser to hang, as
it repeatedly tried to parse an EOF token after hitting an "invalid
destructuring assignment target" error. This change ensures that we
stop parsing after hitting this error condition.

Userland/Libraries/LibJS/Tests/parser-invalid-destructuring-assignment-target.js

@@ -0,0 +1,5 @@
+test("Assigning to an invalid destructuring assignment target should fail immediately", () => {
+    expect(() => {
+        eval("[[function=a{1,}=");
+    }).toThrow(SyntaxError);
+});