Add option in viewer to directly show a plugin page
Testing Done:
Ran `gplugin-gtk4-viewer -p gplugin/native-basic-plugin` and it opened the `basic plugin (C)` page directly.
Ran `gplugin-gtk4-viewer -p asfalskjf` and it exited with an error.
Reviewed at https://reviews.imfreedom.org/r/2148/
/*
* Copyright 2021 Elliott Sales de Andrade <quantum.analyst@gmail.com>
*
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