### Godot version

4.0 (c1128e911ccd6f1e8c35646df804d894652a58f1)

### System information

Arch Linux

### Issue description

Read-only dictionaries can still have new keys added. The value for the new key isn't set (it is kept `null`).

This is caused by `Dictionary::operator[]` not checking if the key exists when it is read only mode. Accessing a key in the underlying HashMap will create if it does not exist. The actual value isn't set because it only returns a dummy reference.

### Steps to reproduce

Run the following snippet:

```gdscript
var dict = {}
dict.make_read_only()
prints("dict", dict) # {}
dict.foo = "hi"
prints("dict", dict) # { "foo": <null> }
```

### Minimal reproduction project

[DictReadOnlyAddKey.zip](https://github.com/godotengine/godot/files/10943387/DictReadOnlyAddKey.zip)

if (_p->variant_map.is_empty()) {
		return;
	}
	for (const KeyValue<Variant, Variant> &E : _p->variant_map) {
		p_keys->push_back(E.key);
	}
}
Variant Dictionary::get_key_at_index(int p_index) const {
	int index = 0;
	for (const KeyValue<Variant, Variant> &E : _p->variant_map) {
		if (index == p_index) {
			return E.key;
		}
		index++;
	}
	return Variant();
}
Variant Dictionary::get_value_at_index(int p_index) const {
	int index = 0;
	for (const KeyValue<Variant, Variant> &E : _p->variant_map) {
		if (index == p_index) {
			return E.value;
		}
		index++;
	}
	return Variant();
}
Variant &Dictionary::operator[](const Variant &p_key) {