home-manager: internalize uninstall

This adds a Boolean option `uninstall`. When enabled this option will
reset side-effecting configurations to their "empty" state. The intent
is that this will cause the activation script to remove all managed
files and packages.

Doing it this way should hopefully be more robust than the previous
solution. It also allows a somewhat more convenient uninstall process
when using Flakes; put `uninstall = true` in your existing
configuration and then do a switch.

Also add simple uninstall test in CI test job.

docs/release-notes/rl-2405.md

@@ -10,6 +10,27 @@ This release has the following notable changes:
 -   The `.release` file in the Home Manager project root has been
     removed. Please use the `release.json` file instead.
 
+- The {command}`home-manager uninstall` command has been reworked to,
+  hopefully, be more robust. The new implementation makes use of a new
+  Boolean configuration option [uninstall](#opt-uninstall) that can
+  also be used in a pure Nix Flake setup.
+
+  Specifically, if you are using a Flake only installation, then you
+  can clean up a Home Manager installation by adding
+
+  ``` nix
+  uninstall = true;
+  ```
+
+  to your existing configuration and then build and activate. This
+  will override any other configuration and cause, for example, the
+  removal of all managed files.
+
+  Please be very careful when enabling this option since activating
+  the built configuration will not only remove the managed files but
+  _all_ Home Manager state from your user environment. This includes
+  removing all your historic Home Manager generations!
+
 ## State Version Changes {#sec-release-24.05-state-version-changes}
 
 The state version in this release includes the changes below. These