Add example for *repl-prompt*

[mpwillson]

I thought an example for this capability might help.
-mark

[sogaiu]

Thanks! I gave it a try by pasting it into a running REPL and it worked fine.

Unfortunately, the instructions for naming example files is out of date.

I think the filename may need to be _42repl-prompt_42.janet for the results to show up on the Core API page. That works here when I test it anyway (^^;

Please rename the file in this PR.

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P.S. Possibly of interest might be this open PR that contains a script for generating an appropriate filename.

[sogaiu]

Thanks!

[mpwillson]

Thanks sogaiu - that was fast!