### 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
token_type = GDScriptTokenizer::Token::IDENTIFIER;
	}
	ParseFunction prefix_rule = get_rule(token_type)->prefix;
	if (prefix_rule == nullptr) {
		
		return nullptr;
	}
	advance(); 
	ExpressionNode *previous_operand = (this->*prefix_rule)(nullptr, p_can_assign);
	while (p_precedence <= get_rule(current.type)->precedence) {
		if (previous_operand == nullptr || (p_stop_on_assign && current.type == GDScriptTokenizer::Token::EQUAL) || (previous_operand->type == Node::LAMBDA && lambda_ended)) {
			return previous_operand;
		}
		
		switch (current.type) {
			
			case GDScriptTokenizer::Token::PARENTHESIS_OPEN:
			case GDScriptTokenizer::Token::BRACKET_OPEN:
				push_multiline(true);
				break;
			default:
				break; 
		}
		token = advance();
		ParseFunction infix_rule = get_rule(token.type)->infix;
		previous_operand = (this->*infix_rule)(previous_operand, p_can_assign);
	}
	return previous_operand;
}
GDScriptParser::ExpressionNode *GDScriptParser::parse_expression(bool p_can_assign, bool p_stop_on_assign) {