...#528)
[`react-rails` now supports other package
managers](reactjs/react-rails#1306) using
`package_json`, meaning it now defaults to `npm` if a package manager is
not explicitly set in `package.json`, causing a stray
`package-lock.json` to be created.
I've specified an exact version because currently `corepack` requires
that ([ranges are not yet
supported](nodejs/corepack#95)) but we're not
actually expecting anyone to be using `corepack` and we're definitely
not using it in production (not explicitly at least...) so in theory
this shouldn't cause problems and it means `package.json` conforms to
the JSON schema - it'll also help to flush out any potential problems
early before(/if) `corepack` becomes more mainstream.