Extensions: Create a new extensions package

Create new library project: @stratosui/extensions

Move some code from core to this new package - minimally code in core/extensions (there's only 2 files there)
May need to move more - e.g. TabNavService
Ensure we don't end up with circular dependencies.
Probably don't want any UI components in this package

Update the build script to also build this library.

Update @stratosui/core to reference @stratosui/extensions. Should also update references in other extensions.

We then should be able to:

• Create a new angular library that is our test extension
• Add that as a dependency in package.json (or in stratos.yaml as an extension to include)

Then we should be able to build the thin app with our extension - extension could just add an item to the left-hand navigation.