### 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_assignment->operation == GDScriptParser::AssignmentNode::OP_NONE && assignee_type.is_hard_type() && p_assignment->assigned_value->is_constant) {
		update_const_expression_builtin_type(p_assignment->assigned_value, assignee_type, "assign");
	}
	GDScriptParser::DataType assigned_value_type = p_assignment->assigned_value->get_datatype();
	bool assignee_is_variant = assignee_type.is_variant();
	bool assignee_is_hard = assignee_type.is_hard_type();
	bool assigned_is_variant = assigned_value_type.is_variant();
	bool assigned_is_hard = assigned_value_type.is_hard_type();
	bool compatible = true;
	bool downgrades_assignee = false;
	bool downgrades_assigned = false;
	GDScriptParser::DataType op_type = assigned_value_type;
	if (p_assignment->operation != GDScriptParser::AssignmentNode::OP_NONE && !op_type.is_variant()) {
		op_type = get_operation_type(p_assignment->variant_op, assignee_type, assigned_value_type, compatible, p_assignment->assigned_value);
		if (assignee_is_variant) {
			
			mark_node_unsafe(p_assignment);
		} else if (!compatible) {
			
			mark_node_unsafe(p_assignment);
			if (assigned_is_variant) {
				
				p_assignment->use_conversion_assign = true;
			} else {
				
				push_error(vformat(R"(Invalid operands "%s" and "%s" for assignment operator.)", assignee_type.to_string(), assigned_value_type.to_string()), p_assignment);
			}
		} else if (op_type.type_source == GDScriptParser::DataType::UNDETECTED && !assigned_is_variant) {
			
			downgrades_assignee = !assignee_is_hard;