Exclude generated tailwind config from VCS

[sedubois]

I had pushed changes to #1280 too quickly, hadn't finished testing. Here is a better fix.

Replaces run with insert_into_file.

Tested as follows:

rails new test-app --database=postgresql && cd test-app
rails db:create
echo 'gem "spina", git: "https://github.com/sedubois/Spina.git", branch: "patch-2"' >> "Gemfile"
bundle
rails active_storage:install
rails db:migrate
rails g spina:install