Move .plugged to addons/gd-plug/.plugged

[Jack-023]

Moving the dir here stops gdscript warnings from triggering on addon code. The Godot team have indicated that they will only support disabling warnings in this directory as it is the only place third party code should be placed.

This change addresses #32, following the discussion there and at godotengine/godot#93889

[imjp94]

Thanks for the contribution!

However, I will have to wait until godotengine/godot#93889 actually merged into Godot master branch.
And, automated migration has to be implemented as well, since it break compatibility with previous version

[Jack-023]

Why does this change depend on godotengine/godot#93889? In the current Godot release you can enable debug/gdscript/warnings/exclude_addons (enabled by default) which will stop anything in res://addons/ from triggering warnings.

I didn't consider automated migration since I had been deleting the .plugged directory after running installs to get rid of the warnings 😅. Migrating is obviously pretty straightforward to do manually but I am not sure how I would go about setting that up in an automated way.

[imjp94]

Oh, I didn't know debug/gdscript/warnings/exclude_addons exist. Then, it's good.

Automated migration is necessary for a smooth user experience, otherwise, the old .plugged will be taking out memory in users' project without any purpose and users may not know it should be removed.
It's ok, I can take over from here. But, I don't have time to work on it yet, so it gonna takes a while before this PR can be merged.

[Jack-023]

FYI in case you decide to use this as a starting point when you pick this up. Something seems to have changed in Godot that has caused this PR to break. It isn't moving files from addons/gd-plug/.plugged after cloning them any more.