This introduces some type hints we can use to fully verify the CSS and

JavaScript bundle configurations used for an extension. These are

effectively supersets of Pipeline's own package configuration formats,

with some additional settings that we offer (such as apply_to and

include_bundles).
Some code working with bundles have been updated to make use of the new

types (and have had type hints introduced as a side-effect). This is not

comprehensive. More code will be updated in coming changes.
This is all part of a dual project to both allow extensions to use

modern static media building support, and to begin incorporating Ink

into the codebase.

Unit tests pass.