libide-gui: switch to GJS for plugins

Christian Hergert requested to merge wip/chergert/gjs-plugin into main

PyGObject does not support GTK 4 much at all, nor does it support Dex in a way we want for moving forward. So instead switch to GJS now that libpeas-2.0 has support for GJS-based plugins.

Fixes #2057 (closed)

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