Setup basic Godot GDExtension for Native Shell

[gumaciel]

This patch initializes the repo as a GDExtension. It includes godot-cpp as a submodule (using the master branch since Godot 4.6.1 stable bindings are not yet available). It creates a basic Shell Godot node via C++ that exposes an execute_command(command) function utilizing popen to act as a native shell executor within Godot. Standard GDExtension and SConstruct boilerplate are included to allow for building.

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PR created automatically by Jules for task 6447467357983652259 started by @gumaciel

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