LibJS: Don't punish large arrays with generic indexed property storage

This patch rethinks the way indexed property storage works:

Instead of having a cut-off point at 200 elements where we always move
to generic property storage, we now allow arrays to stay in simple mode
as long as we don't create a gap/hole larger than 200 elements.

We also simplify generic storage to only have a hash map (for now)
instead of juggling both a vector and a hash map. This is mostly fine
since the vast majority of arrays get to stay simple now.

This is a huge speedup on anything that uses basic JS arrays with more
than 200 elements in them. :^)

Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.shift.js

@@ -25,7 +25,8 @@ describe("normal behavior", () => {
 test("Issue #5884, GenericIndexedPropertyStorage::take_first() loses elements", () => {
     const a = [];
     for (let i = 0; i < 300; i++) {
-        a.push(i);
+        // NOTE: We use defineProperty to prevent the array from using SimpleIndexedPropertyStorage
+        Object.defineProperty(a, i, { value: i, writable: false });
     }
     expect(a.length).toBe(300);
     for (let i = 0; i < 300; i++) {