add ability to install export templates

[definitelyokay]

It would be nice if there was a flag -e or --export-templates for godotenv godot install that would also install the relevant export templates on mac/win/linux.

Additionally, you might also need a separate command just to install the export templates (if you hadn't supplied the flag during installation) and probably one to check if they were installed.

Naturally, this would need to work across each of the 3 major OS's, which means installing the export templates to the correct location. Someone had graciously contributed the support for this to https://github.com/chickensoft-games/setup-godot a while back, and I believe I even ported some of the relevant logic for it to GodotEnv, but it will likely need updates.