Support using `deno` as interpreter for `elm repl` and Document how to do it

[vyuh]

Quick Summary: I found that elm repl --interpreter deno works. It only gives many Uncaught SyntaxError and undefined messages before printing results. Can we have elm repl support deno as interpreter instead of node? May be by passing a proper deno command line to the --interpreter flag.

• Elm: 0.19.1
• Browser: https://deno.land/
• Operating System: Windows 10

Additional Details

• The deno command line interface https://deno.land/manual/getting_started/command_line_interface
• These are the things that I typed into the elm repl --interpreter deno. They worked properly.
  https://gist.github.com/vyuh/d8a360f4cd77a51617e37b9450887010

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[jfmengels]

@vyuh I think it will be useful to the maintainers if you can document some specific use-cases or benefits for enabling the repl to run on deno.

Enabling this will likely require an investment in time and learning in order for them to support a new platform, as well as additional maintenance costs for the future (supporting multiple platforms). Hence, it's likely that this won't get prioritized unless there are clear benefits.