GDExtension: Copy DLL to a temp file before opening

This is done only in the editor and only on Windows, to avoid a file
lock that prevents the original library being updated (e.g. by a
compiler).

When the game runs it will load the original DLL and pick up any
changes, only the editor will stay with the copy (until it is restarted
and create a new copy).

The copy is done in place by prepending a `~` to the original file name,
so dependencies that are loaded with a relative file path still work.
When the library is unloaded the copy file is deleted. The copy is also
marked as hidden to not show up in explorer.

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