FIX: Webpacker install

Current webpacker install, was problematic.

Main issue is that current release of webpacker uses Postcss7.
Tailwind 2 expects Postcss8, and that causes a lot of issues.

Tailwind team has published Tailwind 2 compatible Postcss7 [package](https://tailwindcss.com/docs/installation#post-css-7-compatibility-build)

So in order for the generator to work with Rails 6.1 defaults, this PR
deferes to use `tailwindcss@npm:@tailwindcss/postcss7-compat` package.

By, using this package users don't loose anything it is Tailwind 2.

--------------

There is a way to make it work without using the `Postcss7` package by
using latest `rails-webpacker` beta, but I think it is smarter for now
not to use beta releases.

When rails wepbacker publishes latest with `Postcss8` then we can change
the generator.

Author: dixpac

lib/install/tailwindcss_with_webpacker.rb

@@ -1,14 +1,9 @@
-LATEST_WEBPACKER         = "\"@rails\/webpacker\": \"rails\/webpacker#b6c2180\","
 WEBPACK_STYLESHEETS_PATH = "#{Webpacker.config.source_path}/stylesheets"
 APPLICATION_LAYOUT_PATH  = Rails.root.join("app/views/layouts/application.html.erb")
 
-# Current webpacker version relies on an older version of PostCSS
-# which the latest TailwindCSS version is not compatible with
-gsub_file("package.json", /\"@rails\/webpacker\".*/) { |matched_line| matched_line = LATEST_WEBPACKER }
-
-say "Adding latest Tailwind CSS and postCSS"
-run "yarn add tailwindcss@latest postcss@latest autoprefixer@latest"
-insert_into_file "#{Webpacker.config.source_entry_path}/application.js", "\nrequire(\"stylesheets/application.scss\")\n"
+say "Installing Tailwind CSS"
+run "yarn add tailwindcss@npm:@tailwindcss/postcss7-compat postcss@^7 autoprefixer@^9"
+insert_into_file "#{Webpacker.config.source_entry_path}/application.js", "\nimport \"stylesheets/application\"\n"
 
 say "Adding minimal configuration for Tailwind CSS to work properly"
 directory Pathname.new(__dir__).join("stylesheets"), Webpacker.config.source_path.join("stylesheets")