Add TUI (text-based user interface) for GHCi debugger

GDB has a really nice TUI mode: https://upload.wikimedia.org/wikipedia/commons/4/47/GDB_TUI.png

It's a curses interface which highlights the line number you're on, and also shows the surrounding lines.

GHCi currently allows stepping through with the :step command, which displays line and column numbers. It would be nice to have some visual indication along with these numbers.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 3

added GHCi Tfeature request Trac import labels

IMHO we should remove stuff from the front-end parts instead of adding more. We already have more stuff than we can maintain.

Ideally we'd just provide an API to the users and GHCi would be an app distributed separately.

Ideally we'd just provide an API to the users and GHCi would be an app distributed separately.

See https://github.com/commercialhaskell/intero.

changed title from Add TUI (text-based user interface) for GHCi to Add TUI (text-based user interface) for GHCi debugger

added debugger label

added Plow label

mentioned in issue #15785