Port plugins from libpeas-1 to libpeas-2

The newer version of libpeas was released a couple of years ago already,
and at this point should probably be consdered the only maintained
version.

The changes are pretty minimal, and most should be pretty obvious from
the libpeas migration guide. The only annoying thing is that we can't
use `Peas.Engine.create_extension()` directly, since it's not exposed to
language bindings (even to Vala, which supports varargs, but alas).

Link: https://gnome.pages.gitlab.gnome.org/libpeas/libpeas-2/migrating-1to2.html

Fixes: https://gitlab.gnome.org/GNOME/geary/-/issues/1690

org.gnome.Geary.json

@@ -218,11 +218,19 @@
         {
             "name": "libpeas",
             "buildsystem": "meson",
+            "config-opts": [
+                "-Dgtk_doc=false",
+                "-Dintrospection=true",
+                "-Dvapi=true",
+                "-Dlua51=false",
+                "-Dgjs=false",
+                "-Dpython3=false"
+            ],
             "sources": [
                 {
                     "type": "git",
-                    "url": "https://gitlab.gnome.org/GNOME/libpeas.git",
-                    "commit": "7f245fa5158204621c3469756a951b68a651c4fe"
+                    "url": "https://gitlab.gnome.org/GNOME/libpeas",
+                    "branch": "main"
                 }
             ]
         },