Allow themes/modules to declare libraries via their own *.info.yml

Problem/Motivation

Currently, the only way to declare libraries that Style Selector should attach during rendering is via the module config page. Investigate a way to allow modules and themes to create entries in their own *.info.yml/*.libraries.yml that will be picked up and included as appropriate.

This should also allow removal of style_selector_demo.install.

Proposed resolution

Scan extensions for style_selector keys in *.info.yml, e.g:

style_selector
  shared:
    - mytheme/backgrounds
    - mytheme/foo
  admin:
    - mytheme/bar
  theme:
    - mytheme/baz

Implement a helper method that gathers these "declared" libs and merges with any configured libs.

Update anywhere libs are attached to use the new helper method.

Remaining tasks

• Code.
• Test.
• Merge.

User interface changes

Add a way to list libraries provided by other modules/themes on the module config page.

API changes

None.

Data model changes

None.