fix: explicitly set our preferred package manager for js dependencies (…

…#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.

variants/frontend-base/template.rb

@@ -16,6 +16,11 @@
 # this will create a package.json for us
 run "rails shakapacker:install"
 
+# explicitly set our preferred package manager for external tooling
+update_package_json do |package_json|
+  package_json["packageManager"] = "[email protected]"
+end
+
 # this is added by shakapacker:install, but we've already got one (with some extra tags)
 # in our template, so remove theirs otherwise the app will error when rendering this
 gsub_file "app/views/layouts/application.html.erb",