This function makes it possible to build an Emacs instance that,
instead of launching an Emacs built by Nix, configures an Emacs
already present on the system to use the packages built by Nix.
This **requires** that the versions of the two Emacsen (i.e. the one
used by Nix to build and the one used to run the packages) are kept in
sync, otherwise byte-code incompatibilities may lead to undefined
behaviour.
Exposes an `overrideEmacs` which can take a package function that
receives the current package list and can make arbitrary modifications
to it.
This makes it possible for me to maintain a private overlay for e.g.
work purposes with packages that should not be visible in my public
repos.
This incredible package provides a fully functional, libvterm based
terminal emulator inside of Emacs.
Killer feature: It's possible to switch the buffer into a full Emacs
text mode (read-only) for selections and such.
Builds an Emacs that is not only configured with the required packages
but with the entire Emacs configuration for my personal setup.
This means that `nix-env -iA tools.emacs` will install a
fully-configured Emacs that can be launched as the window manager from
my ~/.xsession.
