### 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 (p_assignable->initializer != nullptr) {
		reduce_expression(p_assignable->initializer);
		if (p_assignable->initializer->type == GDScriptParser::Node::ARRAY) {
			GDScriptParser::ArrayNode *array = static_cast<GDScriptParser::ArrayNode *>(p_assignable->initializer);
			if (has_specified_type && specified_type.has_container_element_type()) {
				update_array_literal_element_type(array, specified_type.get_container_element_type());
			}
		}
		if (is_constant && !p_assignable->initializer->is_constant) {
			bool is_initializer_value_reduced = false;
			Variant initializer_value = make_expression_reduced_value(p_assignable->initializer, is_initializer_value_reduced);
			if (is_initializer_value_reduced) {
				p_assignable->initializer->is_constant = true;
				p_assignable->initializer->reduced_value = initializer_value;
			} else {
				push_error(vformat(R"(Assigned value for %s "%s" isn't a constant expression.)", p_kind, p_assignable->identifier->name), p_assignable->initializer);
			}
		}
		if (has_specified_type && p_assignable->initializer->is_constant) {
			update_const_expression_builtin_type(p_assignable->initializer, specified_type, "assign");
		}
		GDScriptParser::DataType initializer_type = p_assignable->initializer->get_datatype();
		if (p_assignable->infer_datatype) {
			if (!initializer_type.is_set() || initializer_type.has_no_type() || !initializer_type.is_hard_type()) {
				push_error(vformat(R"(Cannot infer the type of "%s" %s because the value doesn't have a set type.)", p_assignable->identifier->name, p_kind), p_assignable->initializer);
			} else if (initializer_type.kind == GDScriptParser::DataType::BUILTIN && initializer_type.builtin_type == Variant::NIL && !is_constant) {
				push_error(vformat(R"(Cannot infer the type of "%s" %s because the value is "null".)", p_assignable->identifier->name, p_kind), p_assignable->initializer);
			}
#ifdef DEBUG_ENABLED
			if (initializer_type.is_hard_type() && initializer_type.is_variant()) {