Add OS::get_cwd virtual

[L2750558108]

It seems that we get current working directory by creating a DirAccess without path init and use get_current_dir(). It doesn't look intuitive.
In fact, the DirAccess finally calls the cwd getting function of specific platform. This PR move them to the OS::get_cwd virtual.

[Calinou]

Please generate documentation by running the compiled editor with --doctool, then fill in the changes made to doc/classes/OS.xml.

[L2750558108]

Please generate documentation by running the compiled editor with --doctool, then fill in the changes made to doc/classes/OS.xml.

Do functions that are not registered in the bind_method also need it?

[bruvzg]

Also take a look at our code style guidelines, there are some issues with it.

[bruvzg]

Note: neither set_cwd, not newly added get_cwd are bound, and therefore it will be accessible only from the engine C++ code (not from GDScript or GDExtension). If you want't it to be accessible, a wrapper should be added to the CoreBind (OS is a special case and its methods can't be bound directly).

Here's links to one of such wrappers, for example (OS.get_name):

Wrapper definition:

godot/core/core_bind.h

Line 175 in 9adb7c7

String get_name() const;

Wrapper implementation:

godot/core/core_bind.cpp

Lines 351 to 353 in 9adb7c7

	String OS::get_name() const {
	return ::OS::get_singleton()->get_name();
	}

Wrapper binding:

godot/core/core_bind.cpp

Line 601 in 9adb7c7

ClassDB::bind_method(D_METHOD("get_name"), &OS::get_name);

[L2750558108]

I noticed that set_cwd is not bound, so I don't plan to get_cwd the corresponding one.

[bruvzg]

I'm not sure what's the purpose of adding get_cwd() if it can't be used from script and not used by the engine.

[L2750558108]

I hope it will provide an intuitive way to get the current working directory in future engine development, and it will correspond to set_cwd.
Maybe it could also be used to gradually replace the original code that used DirAccess to get the working path?

[Delsin-Yu]

Can OS::get_current_working_directory be a better alternative naming-wise? Or is cwd a common knowledge

[L2750558108]

Can OS::get_current_working_directory be a better alternative naming-wise? Or is cwd a common knowledge

The name comes from another set_cwd method that already exists. I'm not sure if this should be changed

[Delsin-Yu]

I'm not sure if this should be changed

Oh, then no.