### Godot version

4.1.beta3

### System information

Fedora Workstation 38 - Godot 4.1.beta3 - Vulkan (Forward+)

### Issue description

**This is an issue created by the production team, we're working on it**

Currently, it's impossible to call virtual functions within GDScript.

This currently returns an error:
```gdscript
# a.gd
extends Node

func _process(delta: float) -> void:
	pass
```

```gdscript
# main.gd
extends Node

var my_node: Node = preload("res://a.gd").new()

func _process(delta: float) -> void:
	my_node._process(delta)  # Cannot call virtual function "_process()" because it hasn't been defined.
```

### Steps to reproduce

1. Create a script (`a.gd`) based on `Node` that declares `_process()`;
2. Create a main node;
3. Attach a script to the main node;
4. Initialize a variable with a new `a.gd` instance;
5. Try to run `_process()` on that node;
6. Find out that it returns an error.

### Minimal reproduction project

[virtual_functions.zip (updated)](https://github.com/godotengine/godot/files/11823992/virtual_functions.zip)


if (subscript->base->type == GDScriptParser::Node::IDENTIFIER) {
			base_id = static_cast<GDScriptParser::IdentifierNode *>(subscript->base);
		}
		if (base_id && GDScriptParser::get_builtin_type(base_id->name) < Variant::VARIANT_MAX) {
			base_type = make_builtin_meta_type(GDScriptParser::get_builtin_type(base_id->name));
		} else {
			reduce_expression(subscript->base);
			base_type = subscript->base->get_datatype();
			is_self = subscript->base->type == GDScriptParser::Node::SELF;
		}
	} else {
		
		
		p_call->set_datatype(call_type);
		mark_node_unsafe(p_call);
		return;
	}
	int default_arg_count = 0;
	BitField<MethodFlags> method_flags;
	GDScriptParser::DataType return_type;
	List<GDScriptParser::DataType> par_types;
	bool is_constructor = (base_type.is_meta_type || (p_call->callee && p_call->callee->type == GDScriptParser::Node::IDENTIFIER)) && p_call->function_name == SNAME("new");
	if (get_function_signature(p_call, is_constructor, base_type, p_call->function_name, return_type, par_types, default_arg_count, method_flags)) {
		
		
		if (method_flags.has_flag(METHOD_FLAG_VIRTUAL)) {
			if (p_call->is_super) {
				push_error(vformat(R"*(Cannot call the parent class' virtual function "%s()" because it hasn't been defined.)*", p_call->function_name), p_call);
			} else {
				push_error(vformat(R"*(Cannot call virtual function "%s()" because it hasn't been defined.)*", p_call->function_name), p_call);