Draft: Importmap experiment

Still not completely working. Bootstrap JS controls are working, but Arclight-specific ones don't seem to be. Just a starting point as we explore how to work with import maps.

Both Arclight and Blacklight's install generators add to config/importmap.rb if you're detected to be using ImportMaps. In experimental, we are, but that file didn't exist so the generators never appended to it (or they never ran?) Here I've created importmap.rb as I think it would be if a new Arclight project was initialized from scratch (but I didn't actually initialize one from scratch and copy the file over, so there could be mistakes.

I tried to do the same with application.js, but what I came up with printed errors to the JS console: Arclight's JS complained that Stimulus was undefined, so I had to inline arclight.js into application.js and explicitly initialize Stimulus. I assume this is not the correct solution.