A Component that will start/stop REPL servers based on environment variables.

REPL_PORT - if set, start a Socket REPL server on this port

DEV_REPL_PORT - if set, start an optionally CIDER-enabled nREPL server on this port

The latter requires that cider/cider-nrepl and refactor-nrepl are on the
classpath. They are require'd at runtime.

(env-repl/system) will return a Component; calling start on it will cause REPL
servers to be started for each of the above environment variables, if set;
calling stop on it will stop those REPL servers, if they are known to be
started.

(env-repl/system 'repl-name) will return a Component that will use repl-name
as the name for the Socket REPL, if it starts one. The default is 'repl