Add application.js if missing

[jlugner]

As mentioned in #252 rails 6+ doesn't generate the required javascript entry point for godmins install_generator. Below approach is likely an overly naive solution but should do the trick for projects running rails 6+. I've done some basic testing for both the Engine installation and standalone installations for latest versions of rails 6 and 7, and it seems to work.

[jlugner]

Failing build seems to be Capybara not playing well with the recent release of Puma 6.0. Fixed on Capybara#master, so probably overkill to lock the version here. New version should be out soon.