This creates a custom logger for the log crate which logs everything to a file (/tmp/starship/session_$STARSHIP_SESSION_KEY.log) and it logs everything above Warn to stderr, but only if the log file does not contain the line that should be logged resulting in an error or warning to be only logged at the first starship invocation after opening the shell.
Shell inits are now in a separate directory in the source code, with each shell getting its own script. Also adds a little DRY + commenting in init/mod.rs.
