### 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
IdentifierNode *id = static_cast<IdentifierNode *>(p_previous_operand);
			switch (id->source) {
				case IdentifierNode::LOCAL_VARIABLE:
					source_variable = id->variable_source;
					id->variable_source->usages--;
					break;
				case IdentifierNode::LOCAL_CONSTANT:
					id->constant_source->usages--;
					break;
				case IdentifierNode::FUNCTION_PARAMETER:
					id->parameter_source->usages--;
					break;
				case IdentifierNode::LOCAL_ITERATOR:
				case IdentifierNode::LOCAL_BIND:
					id->bind_source->usages--;
					break;
				default:
					break;
			}
#endif
		} break;
		case Node::SUBSCRIPT:
			
			break;
		default:
			push_error(R"(Only identifier, attribute access, and subscription access can be used as assignment target.)");
			return parse_expression(false); 
	}
	AssignmentNode *assignment = alloc_node<AssignmentNode>();
	reset_extents(assignment, p_previous_operand);