allow {file} when --partition-size is 1

Scripts would read easier if I could use {file} instead of {files} when partition size is 1.

I've implemented this (along with some other changes) in my fork.
See the quote at the top of the readme for more info.

https://www.npmjs.com/package/@aleclarson/bucket-runner

npm i -g @aleclarson/bucket-runner

pnpm i -g @aleclarson/bucket-runner

yarn global add @aleclarson/bucket-runner