### Godot version

4.0 stable

### System information

Fedora, RX 590

### Issue description

I made a dictionary, and instead of using `:` to separate between key and value, i accidentally used `=`.
The error message was not clear at all `Parse Error: Expected identifier or string as LUA-style dictionary key.` 

I don't think assuming that people know how dictionaries are made in LUA is reasonable. And if this is not about LUA the programming language, i don't even know what this is about :D

I think this error should be replaced with something of the sort "Please use `:` as separator instead of `=` in your dictionary"

### Steps to reproduce

Paste this code in a script:

```

var dict = {
	1 = "lala"
}
```

Observe the error message
![image](https://user-images.githubusercontent.com/7917475/224549538-38ea25aa-ba75-431e-868b-0733a53b90ad.png)


### Minimal reproduction project

see above
ExpressionNode *grouped = parse_expression(false);
	pop_multiline();
	if (grouped == nullptr) {
		push_error(R"(Expected grouping expression.)");
	} else {
		consume(GDScriptTokenizer::Token::PARENTHESIS_CLOSE, R"*(Expected closing ")" after grouping expression.)*");
	}
	return grouped;
}
GDScriptParser::ExpressionNode *GDScriptParser::parse_attribute(ExpressionNode *p_previous_operand, bool p_can_assign) {
	SubscriptNode *attribute = alloc_node<SubscriptNode>();
	reset_extents(attribute, p_previous_operand);
	update_extents(attribute);
	if (for_completion) {
		bool is_builtin = false;
		if (p_previous_operand && p_previous_operand->type == Node::IDENTIFIER) {
			const IdentifierNode *id = static_cast<const IdentifierNode *>(p_previous_operand);
			Variant::Type builtin_type = get_builtin_type(id->name);
			if (builtin_type < Variant::VARIANT_MAX) {
				make_completion_context(COMPLETION_BUILT_IN_TYPE_CONSTANT_OR_STATIC_METHOD, builtin_type, true);
				is_builtin = true;
			}
		}
		if (!is_builtin) {
			make_completion_context(COMPLETION_ATTRIBUTE, attribute, -1, true);
		}