This lets us take advantage of nix strings while having the entire
config in a single portable file.
Note that someone already wrote a home manager module for joshuto, which
should get merged soon.
See: https://github.com/nix-community/home-manager/pull/4004
Note that the theme file is necessary to avoid the theme changing in
certain situations. The keymap config is the same as the default, except
with the addition of "o", which is used to select files (or a directory)
when using joshuto as a file chooser.
Since all of these files do roughly the same thing (that is, configure
the system in a specific way that a separate file seems necessary), this
should reduce the overall complexity of the project tree.
