### 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)


case GDScriptParser::IdentifierNode::UNDEFINED_SOURCE:
			break;
	}
	
	if (!found_source) {
		reduce_identifier_from_base(p_identifier);
		if (p_identifier->source != GDScriptParser::IdentifierNode::UNDEFINED_SOURCE || p_identifier->get_datatype().is_set()) {
			
			found_source = true;
		}
	}
	if (found_source) {
		bool source_is_variable = p_identifier->source == GDScriptParser::IdentifierNode::MEMBER_VARIABLE || p_identifier->source == GDScriptParser::IdentifierNode::INHERITED_VARIABLE;
		bool source_is_signal = p_identifier->source == GDScriptParser::IdentifierNode::MEMBER_SIGNAL;
		if ((source_is_variable || source_is_signal) && static_context) {
			if (parser->current_function) {
				
				GDScriptParser::FunctionNode *parent_function = parser->current_function;
				while (parent_function->source_lambda) {
					parent_function = parent_function->source_lambda->parent_function;
				}
				push_error(vformat(R"*(Cannot access %s "%s" from the static function "%s()".)*", source_is_signal ? "signal" : "instance variable", p_identifier->name, parent_function->identifier->name), p_identifier);
			} else {
				push_error(vformat(R"*(Cannot access %s "%s" for a static variable initializer.)*", source_is_signal ? "signal" : "instance variable", p_identifier->name), p_identifier);
			}
		}
		if (!lambda_stack.is_empty()) {
			
			if (source_is_variable || source_is_signal) {
				mark_lambda_use_self();