.prettierignore for monorepos

Workaround for Prettier's inability to use nested ignore-files.

Provides prettierignore-monorepo bin that:

• Collects all .gitignore and .prettierignore files in the monorepo
• Glues them into a single root .prettierignore
• Appends rules from the root .prettierignore-append (optional)

Usage

• Install as a dev dependency in the monorepo root
• If .prettierignore exists in the root, rename it to .prettierignore-append (and git rm --cached .prettierignore)
• Add /.prettierignore to root .gitignore
• Run prettierignore-monorepo bin to generate initial .prettierignore
• Set up the prettierignore-monorepo bin to run on some event (e.g. on a git hook, or in the postinstall script of the root package.json)