SI-7805 REPL -i startup

Tested with a ReplTest that loads an include script.

ReplTests can choose to be `Welcoming` and keep a
normalized welcome message in their check transcript.

One recent SessionTest is updated to use the normalizing API.
Merge pull request #2917 from som-snytt/issue/7805-repl-i

SI-7805 REPL -i startup