Add const iteration support to Dictionary

[YYF233333]

Based on the assumption in #103999, we are able to bring ConstIterator to Dictionary without breaking the read_only property. Since the inner HashMap already has a nice iterator implementation, I just export it with alias to keep the change minimal.

Main usecase of this is to replace Dictionary::get_key_list(). Previously if we want to iterate a Dictionary we have to write:

List<Variant> keys;
dict.get_key_list(&keys);
for (const Variant &key : keys) {
    const Variant &value = dict[key];
    ......
}

This pattern includes unnecessary heap allocation (the List), redundant iteration (populate keys in get_key_list()) and an extra hashmap lookup (if you need both key and value).

With ConstIterator we can now write:

for (const KeyValue<Variant, Variant> &kv : dict) {
    const Variant &key = kv.key;
    const Variant &value = kv.value;
    ......
}

No heap allocation, scan exactly one time and no hashmap lookup.

I measure performance diff between the two pattern with MRP from #100333 (Note this workload is constructed intentionally to be bottlenecked at Dictionary iteration, Dictionary stringify would be a better showcase but unfortunately it is now bottlenecked by String concat)

Master:

729 ms
732 ms
729 ms
729 ms
727 ms
729 ms
725 ms
725 ms
720 ms
714 ms
average: 725 ms

This PR:

271 ms
270 ms
273 ms
271 ms
271 ms
272 ms
271 ms
273 ms
271 ms
272 ms
average: 271 ms

Callers who sort the key list or modify values in the loop are kept untouched.

[Repiteo]

Only needing a const iterator completely sidesteps the readonly hurdles, clever! I was a bit hesitant on exposing HashMap in the header, but we're already doing that with List so I suppose that can be dealt with when we start the clangd passes.

[Repiteo]

Might be jumping the gun a bit here, but I'm confident enough about dictionaries to send this as-is.

Thanks!