At connection, a remote client sends all its environement to the
server, which then provides access to client env through
kak_client_env_VAR_NAME variables in the shell.
Now %sh{ ... } can access positional parameters through
standard shell construct, allowing simple handling of variable
parameters.
def command accepts -shell-params flag to use this facility.
when a kakoune releated env var used in a shell command, the ShellManager
tries to match it with given regex and the first that matches calls its
value retriever.
For this to be useful, EnvVarRetrievers now also take the variable
name in its parameters.
