### 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_from._p->refcount.ref()) {
		return; 
	}
	
	if (p_from._p == _p) {
		_p->refcount.unref();
		return;
	}
	if (_p) {
		_unref();
	}
	_p = p_from._p;
}
void Dictionary::clear() {
	ERR_FAIL_COND_MSG(_p->read_only, "Dictionary is in read-only state.");
	_p->variant_map.clear();
}
void Dictionary::merge(const Dictionary &p_dictionary, bool p_overwrite) {
	for (const KeyValue<Variant, Variant> &E : p_dictionary._p->variant_map) {
		if (p_overwrite || !has(E.key)) {
			this->operator[](E.key) = E.value;
		}
	}
}
void Dictionary::_unref() const {