As we discussed in bug bash.
There's really no downside to us enabling it by default (and leaving
showMarksOnScrollbar: false). It'll mark lines as "prompts" when the
user hits enter. This will have a couple good side effects:
* When folks have right-aligned prompts (like, from oh-my-posh), the
`enter` will terminate where shell integration thinks the command is, so
that the right-prompt doesn't end up in the commandline history
* the scrollToMark actions will Just Work without any other shell
integration
Closes#17632
fd1b1c35b4