Enhanced mobile tabContainer + navigator routing support

[amcclain]

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[amcclain]

Matching the config from tabContainerModel - allow dev to scope down routing responsiveness to a particular sub-path.

[amcclain]

Goal here is to allow use of TabContainer and Navigator within nested sub-routes. Specifically looking to support an overall UI pattern where we have a top-level tab container, switchable primarily via a bottom tab-bar, and within each top-level tab we have a swipable, navigator-powered page stack.

Possible TODOs:

• TabContainer should swap back to last-visited route within that tab when re-activated - i.e. maintain state as we expect with desktop.
• Tapping the switcher button for an already-active tab should navigate to its base/default route

[amcclain]

NOTE - had a good discussion with @ghsolomon and @oodarluz today and we are going to look to double-down on navigator only, allowing it to support the end-user experience I was trying to achieve here, without developers needing to setup multiple navigators and pair up with tabContainer.

See #3985 - evolving spec to capture the plan. Leaving this PR open in draft for the moment, but expect to close out this branch without merging in the near future.