Add hint for :quit to REPL.REPLCompletions.UndefVarError_hint

[mkitti]

This addresses #38522 (comment) by providing a hint when a user types quit or quit() into the REPL.

It follows on the heels of #41754 that added support for when the user types help.

julia> using Revise, REPL

julia> Revise.track(REPL)

julia> quit
ERROR: UndefVarError: quit not defined
suggestion: To exit Julia, use Ctrl-D, or type exit() and press enter.

julia> quit()
ERROR: UndefVarError: quit not defined
suggestion: To exit Julia, use Ctrl-D, or type exit() and press enter.
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

julia> quit = 2
2

julia> quit
2

julia> quit()
ERROR: MethodError: objects of type Int64 are not callable
Maybe you forgot to use an operator such as *, ^, %, / etc. ?
Stacktrace:
 [1] top-level scope
   @ REPL[7]:1

[DilumAluthge]

This is really nice!

[mkitti]

Bumping this. It's a straightforward addition to the REPL when a user tries to "quit".

[DilumAluthge]

I think this is good to go.

[DilumAluthge]

Let's merge this. If there are further tweaks to make, we can do so in a follow-up PR.