CharString implementation doesn't match Godot's

[dsnopek]

Godot version

v4.1.beta.custom_build.19c1d3c7a

godot-cpp version

4.1.dev (1bc9ca7)

System information

Linux

Issue description

The CharString implementation in godot-cpp is much simpler, and not compatible with what's in Godot. This makes it harder to have a module that can also be compiled as a GDExtension.

This was discovered when trying to compile the text_server_adv module in Godot as a GDExtension.

See: godotengine/godot#77532

Steps to reproduce

# Assuming you have Godot's source code in 'godot'
cd godot/modules/text_server_adv/gdextension_build
scons

Minimal reproduction project

n/a