REPL: Improved REPL, redux

[SethTisue]

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This ticket replaces https://github.com/scala/scala-dev/issues/325 from 2017. Goals: omit completed tasks, omit less-likely todos, make it easier to see the big picture & see where things stand.

Note that all the https://github.com/scala/scala-dev/labels/t%3Arepl tickets that we opened during the 2.13.3 cycle now live at https://github.com/scala/scala-dev/labels/t%3Arepl

The prioritization here is based on part on the Twitter poll I did, as well as many years of discussions within the team and among the community.

Modest goals

• [ ] Syntax highlighting (https://github.com/scala/bug/issues/12273)
• [ ] Display Scaladoc (or at least links to Scaladoc) (https://github.com/scala/bug/issues/12275)
• [ ] Don't always fully qualify type names in output (https://github.com/scala/bug/issues/8349)

More ambitious

These remain possibilities for Scala 2, but would need discussion/planning. It would make the most sense to land it in Scala 3 first, then backport.

• [ ] Pretty-printing of results (like Ammonite? adopt their solution?)

Shelved waiting for JLine fix

• Tab completion grouping and ordering (https://github.com/scala/bug/issues/12281)
  • sigh, JLine is broken; see remarks on ticket

Out of scope

• Get rid of our own REPL, adopt Ammonite wholesale
  • Perhaps this could happen in Scala 3; I don't know. Certainly not happening in Scala 2: too much work, too disruptive.
  • I would encourage people to instead consider porting Ammonite features to the stock REPL, or to turn Ammonite into a wrapper or client of the stock REPL, to avoid code duplication.
• other possibilities originally included on https://github.com/scala/scala-dev/issues/325
  • ...are probably out of scope for the core team, but contributors might be interested?
• "Magic imports" of dependencies, a la Ammonite's Ivy imports?
  • Overlaps with existing :require feature
  • Involves integrating with Coursier
  • May overlap with making our launchers Coursier-based
  • Moved to "out of scope" with the advent of https://scala-cli.virtuslab.org

[SethTisue]

Current status is that I intend to finish "syntax highlighting" but it appears increasingly unlikely I'll get to the others.

Hence, unassigning myself here but keeping myself assigned to https://github.com/scala/bug/issues/12273