Support building wpcarros-emacs

I'm using a Makefile until I can remember the command:

```shell
> nix-env -f . -i
```

This will install (i.e. `-i`) any derivations instantiated from the Nix
expression resolvable by `-f`. Ideally the incantation will look something like
this:

```shell
> nix-env -f '<universe>' -iA emacs
```

Informing `nix-env` to install all of the derivations created by the expression
at attribute `emacs` in my `<universe>` repository. For now two things are
preventing this:

1. `emacs` isn't an attribute in my top-level expression defined in the
   `default.nix`.
2. If I do add `emacs` as an attribute and call the above command, my usage of
   `readTree` results in `pkgs` missing `.lib` and a few other stdlib commands
   that are available in `(import <nixpkgs> {})`.

A fix for both of these should be forthcoming.

configs/shared/.xsessionrc.shared

@@ -26,5 +26,6 @@ unclutter &
 # Control X's pointer with the keyboard. Activate by pressing <C-;>
 keynav daemonize
 
-# Startup Emacs (and EXWM).
-exec emacs --debug-init --no-site-file --no-site-lisp
+# Startup Emacs (and EXWM). This is built by calling `make install` from within
+# ~/dotfiles/emacs.
+exec wpcarros-emacs

emacs/Makefile

@@ -0,0 +1,5 @@
+install:
+	# TODO: I'd rather support `nix-env -f '<universe>' -iA emacs`, but at the
+	# moment this fails. I think it's failing because of the way I'm using
+	# readTree. I will need to investigate this further.
+	nix-env -f . -i

emacs/default.nix

@@ -36,6 +36,12 @@ let
       neotree # TODO: Remove this dependency from my config.
       which-key
       ivy
+      restclient
+      package-lint
+      parsec
+      magit-popup
+      direnv
+      emr
       ivy-prescient
       all-the-icons
       all-the-icons-ivy