Expand CI matrix to run tests against a rails 7 app with propshaft

New rails 7 applications can be generated with propshaft as an
alternative asset pipeline to sprockets:
rails/rails@fb1ab34
Since we want importmap-rails to work out of the box with either of
those asset pipeline options, we'll run our test suite continuously
against both.

Author: rmacklin

.github/workflows/ci.yml

@@ -9,6 +9,7 @@ jobs:
           - "3.0"
         gemfile:
           - Gemfile
+          - gemfiles/rails_7_propshaft.gemfile
           - gemfiles/rails_7_sprockets.gemfile
         continue-on-error: [ false ]