### 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)

ERR_FAIL_COND(!_p);
	if (_p->refcount.unref()) {
		if (_p->read_only) {
			memdelete(_p->read_only);
		}
		memdelete(_p);
	}
	_p = nullptr;
}
uint32_t Dictionary::hash() const {
	return recursive_hash(0);
}
uint32_t Dictionary::recursive_hash(int recursion_count) const {
	if (recursion_count > MAX_RECURSION) {
		ERR_PRINT("Max recursion reached");
		return 0;
	}
	uint32_t h = hash_murmur3_one_32(Variant::DICTIONARY);
	recursion_count++;
	for (const KeyValue<Variant, Variant> &E : _p->variant_map) {
		h = hash_murmur3_one_32(E.key.recursive_hash(recursion_count), h);
		h = hash_murmur3_one_32(E.value.recursive_hash(recursion_count), h);
	}
	return hash_fmix32(h);
}
Array Dictionary::keys() const {