plugins: add top-level deprecation file

- Initialize it with the `treesitter-playground` removal

plugins/deprecation.nix

@@ -0,0 +1,22 @@
+{ lib, ... }:
+let
+  removed = {
+    # Added 2023-08-29
+    treesitter-playground = ''
+      The `treesitter-playground` plugin has been deprecated since the functionality is included in Neovim.
+      Use:
+      - `:Inspect` to show the highlight groups under the cursor
+      - `:InspectTree` to show the parsed syntax tree ("TSPlayground")
+      - `:PreviewQuery` to open the Query Editor (Nvim 0.10+)
+    '';
+  };
+in
+{
+  imports = lib.mapAttrsToList (
+    name:
+    lib.mkRemovedOptionModule [
+      "plugins"
+      name
+    ]
+  ) removed;
+}