Support EDITOR and VISUAL environment variables where applicable

[tlmii]

I digress slightly from the original issue but I would've expected setting environment variables $EDITOR/$VISUAL to work as is the case of a lot of CLI tools, e.g., https://git-scm.com/book/en/v2/Customizing-Git-Git-Configuration. If I didn't read the docs, the first thing I would have tried is $EDITOR="code -w" (which is already set in my bash profile) to just work.

Originally posted by @shirhatti in https://github.com/dotnet/HttpRepl/issues/466#issuecomment-754182369