Disable lock files

[PR comment](https://github.com/smeijer/unimported/pull/17#discussion_r568536262)

I know there is an `.npmrc` setting for npm, not sure what the yarn equivalent is, but I'm sure it must be possible to disable the yarn lock file too.

[For reference](https://github.com/testing-library/react-hooks-testing-library/blob/master/.npmrc#L1)