code: store Array.isArray(types[arg]) for reuse
code: store Array.isArray(types[arg]) for reuse

Credit: da39a3ee5e6b4b0d3255bfef95601890afd80709@elidoran
Reviewed-By: da39a3ee5e6b4b0d3255bfef95601890afd80709@othiym23
PR-URL: https://github.com/npm/nopt/pull/75

npm/nopt