* Make Sprockets more optional, offer Propshaft as alternative

* Whups, local reference

* No longer used

* Spacing

* Need explicit sprockets-rails inclusion now

* Manually require the sprockets railtie

* Don't need these changes right now

* Kick off another build

* Fix tests

* DRY up test

* Require railtie when using sprockets

* Introduce option to skip asset pipeline

* No longer relevant

* Always have to return

* Gone

* Add helper for skip_sprockets?

* Fix guard statement

* Use latest gems

* Include propshaft

* fix tests for #43261 (#43277)

* help fix tests for #43261

skip_sprockets? should not be called on options

:skip_sprockets is no longer a value in the option hash, so
skip_sprockets? should not be called on it

move --asset-pipeline to shared generator

skip_sprockets? is defined on app_base, and used in the plugin
generator to determine whether to add the engine's assets to the dummy
sprockets manifest, so I believe it makes sense to include in both
generators

Because of this change, I also changed the shared test back to testing
against non-sprockets

add skip_sprockets to Gemfile template vars

Mocking skip_sprockets? in app_base generator

fix more generator tests

* use skip_sprockets? everywhere

* Use latest propshaft

* Update `AssetUrlHelper` docs to list both asset pipeline gems (#43328)

* Update to latest Propshaft

* Bump Propshaft again

* Ask for latest

* Use latest propshaft

Co-authored-by: Hartley McGuire <skipkayhil@gmail.com>
Co-authored-by: Richard Macklin <1863540+rmacklin@users.noreply.github.com>