When jq is available, it's actually faster to invoke jq and parse the `package.json`
invoking the `npm` command.

Also, prior to this commit, both `__fish_complete_npm` and `__fish_npm_run` were being run
whenever completions for `npm run` subcommand was being used, which was actually making
repetitive work (invoking npm command twice). This pull request is supposed to make completion
without `jq` faster as well

Created function to handle both cases of npm run completion parse, with or without `jq` completion.