Use repl.send-signal to interrupt stdio REPLs

[russtoku]

This is a minor nit but I noticed that the stdio-based clients use two equivalent implementations of the interrupt function. This PR makes the implementation the same for these clients:

• Hy
• Julia
• Python
• Racket

by using the:

• send-signal function defined in remote/stdio.fnl
• vim.loop.constants.SIGINT constant instead of the number 2
• comment-prefix of the client's language

Hopefully, this provides consistency and emphasizes the use of the base client functionality of remote/stdio.fnl.

I didn't include the Scheme client because there is #498 which adds the ability to interrupt the REPL.

[Olical]

Okay, I think that's all of your changes merged to develop fairly cleanly. Please let me know if something seems off! I'm going through myself too.

[russtoku]

All seems in order. Thanks!