Adding Asset Mapper support + new StimulusBundle

[weaverryan]

Q	A
Bug fix?	no
New feature?	yes
Tickets	None
License	MIT

Adds:

• A) Integration with new symfony.importmap key introduced to Flex in New "add-lines" configurator for simple file patching + importmap support flex#975
• B) An "asset mapper" path in each bundle so that you can refer to the controller/files in that bundle.
• C) Introduces new StimulusBundle (thanks to @jmsche for helping port from WebpackEncoreBundle). This includes the Twig stimulus_* functions + a revamped "helper" system for generating those attributes.
• D) The new StimulusBundle also contains integration for AssetMapper to load custom controllers and controllers.json.

Cheers!

[weaverryan]

This is ready for review - test failures are expected - the "low deps" aren't quite smart enough in the CI to know how to handle dependencies against packages that were added/updated inside the same PR. The "high deps" handle it correctly.