Adds a build script using ASDF's program-op to build an executable out
of the Gemma source code.
In addition a Nix derivation is provided that will both compile the
Elm source and place it in a folder, as well as create the executable.
Currently static file serving does not function as intended.
Uses the cl-prevalence system to store tasks on disk. The storage
location is either relative to the working directory in which the
system is started or determined (with priority) by the environment
variable `GEMMA_DATA_DIR`.
