Consider extracting showInvisibles / generateDifferences to a new package

[BPScott]

I recently made https://github.com/BPScott/stylelint-prettier, which does pretty much the same as eslint-plugin-prettier but for stylelint. @ikatyang has https://github.com/ikatyang/tslint-plugin-prettier which is the same for tslint.

Both of us rely upon the reporting api added in #41, but it seems somewhat odd for for us to depend upon a plugin for a different linting tool to get our functionality.

Would it be worth extracting out the showInvisibles and generateDifferences functions into a new prettier-lint-reporting-utils library (but with a better name)?

[BPScott]

https://github.com/prettier/prettier-linter-helpers is now a thing that exists, and shall be used in v3 (#113) 🎉