RFC: Allow prefixing result evaluated at GHCi prompt

Similar to prompt, prompt-cont (previously prompt2): allowing users to supply a prefix to evaluation results with result has two use cases I can think of

:set prompt      "    ghci> "
:set prompt-cont "    ghci| "
:set result      "    "

the desired outcome would be ↓ as indented code for IRC, also reddit code format:

    ghci> 1 + 2 + 3
    6
    ghci> reverse "hello"
    "olleh"

---

The other case came from this tweet

:set prompt      ""
:set result      "-- "

1 + 2 + 3
-- 6
reverse "hello"
-- "olleh"

This can be copied/pasted back to a separate GHCi session where the result is ignored.

---

I haven't seen this feature in any other repl, I'm not sure if this would see much use.

This wouldn't work with IO actions without some magic.

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	lowest
Resolution	Unresolved
Component	GHCi
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