feat(wayland): Add display manager greetd and greeter tuigreet

Sets up a login screen using tuigreet. From there the user is thrown
into a shell. Other units/services (usually, the user service `river`)
take care to start an actual wayland session for the user.

books/templates/greetd-config.toml.j2

@@ -0,0 +1,18 @@
+[terminal]
+# The VT to run the greeter on. Can be "next", "current" or a number
+# designating the VT.
+vt = 7
+
+# The default session, also known as the greeter.
+[default_session]
+command = "tuigreet --cmd zsh"
+user = "{{ greeter_user }}"
+
+# `agreety` is the bundled agetty/login-lookalike. You can replace `/bin/sh`
+# with whatever you want started, such as `sway`.
+# command = "agreety --cmd /bin/sh"
+
+# The user to run the command as. The privileges this user must have depends
+# on the greeter. A graphical greeter may for example require the user to be
+# in the `video` group.
+#user = "_greeter"