Update README.md to use deleteAsync

[bramp]

For ~two years 'del' no longer exports a default method, and instead exports deleteAsync() and deleteSync(), the former replacing del()

See sindresorhus/del@106d7d8

[yocontra]

This seems like a good opportunity to eliminate a dependency and use import fs from 'fs/promises' + rmdir since this example was written before fs/promises existed del was the cleanest to use