[Feature] Import Maps support

[hrdwdmrbl]

Related to #1162 but not identical, I'm interested in using react-rails with Import Maps. It looks like https://github.com/rails/importmap-rails is actually compatible with Rails 6 as well.

[justin808]

@hrdwdmrbl I'll open if you have a compelling use case. Just because Rails supports this for other uses cases does not mean that we should increase the complexity of this project.

[hrdwdmrbl]

@justin808 I’d switch over completely cause it’s now the default in Rails 7