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/