Smart Trim Tokens for text with summary fields

I'm sure the implementation between D7 and D10 is a little different. But a smart trim token for summary fields is actually a missing feature-set in D10 too. And since the life-support for D7 is so short at this point, I'm going to just move versions and work on a version of these functionality for the 2.1.x version.

heddn → changed the visibility of the branch 2.1.x to hidden.

Initial implementation in smart trim 2.1.x provided.

Works as expected, though I'm not sure about the assumption of a `token` view mode -- but I think that's more of a foundational approach with "how do we handle tokens that might need to respond to settings?".

The 'token' view mode logic is exactly what token.module assumes. It seems like that assumption was a safe one to continue using here too.