add 'command type' introspective handler

add 'command type' introspective handler

Adds the 'command' group handler, with the 'type' command producing
a string that tells whether the given command is 'native' (for Jim-based
command handlers), 'simple' (for simple built-in commands), 'group'
for command group placeholders, and 'unknown' if not found in the
command registration tables (e.g. core built-ins functions).