maintainers-list: create a field isActive

Default true; should be set to false when the maintainer is not active.

The most immediate use of this Boolean attribute is to mark maintainers
that are not contributing to Nixpkgs from a long amount of time.
Nonetheless it can be used to mark any *long term inactivity*, including
but not limited to:

- Sabbatical leave
- Retirement
- Intimate issues
- Force majeure

In principle, a third person can set this attribute; nonetheless, for
the sake of a good etiquette, at least one contact attempt must be
issued and carried out before effectively committing it.

This attribute can be employed to many useful activities, including but
not limited to:

- Treewide automation
  - Removal of inactive maintainers from packages
  - Orphaning alerts
- Filter automatic notifications

maintainers/maintainer-list.nix

@@ -13,6 +13,33 @@
       keys = [{
         fingerprint = "AAAA BBBB CCCC DDDD EEEE  FFFF 0000 1111 2222 3333";
       }];
+
+      # Default true; should be set to false when the maintainer is not active.
+      #
+      # The most immediate use of this Boolean attribute is to mark maintainers
+      # that are not contributing to Nixpkgs from a long amount of time.
+      # Nonetheless it can be used to mark any *long term inactivity*, including
+      # but not limited to:
+      #
+      # - Sabbatical leave
+      # - Retirement
+      # - Intimate issues
+      # - Force majeure
+      #
+      # In principle, a third person can set this attribute; nonetheless, for
+      # the sake of a good etiquette, at least one contact attempt must be
+      # issued and carried out before effectively committing it.
+      #
+      # This attribute can be employed to many useful activities, including but
+      # not limited to:
+      #
+      # - Treewide automation
+      #   - Removal of inactive maintainers from packages
+      #   - Orphaning alerts
+      # - Filter automatic notifications
+      #
+
+      isActive = true;
     };
     ```