add: link to replit run

[hyperupcall]

i think it would be pretty cool to let people see how this code works (and edit / preview the program) right in their browser, without any manual downloads or installation. i'm thinking we can add a 'run on repl.it' button, which redirects to something like this:

[ljharb]

We'd not want to link to something that uses an unofficial package installer, so this is a great idea if you can use npm instead of yarn (ie, if you can make the repl.it actually match this repo, which uses npm)

[hyperupcall]

oh gotchia - yes i can match the commands

edit: how does it look

[ljharb]

@eankeen did you delete your fork? :-( if so, could you try recreating it, so I can land this?

[ljharb]

@eankeen I'd still like to land this, but it appears your fork has been deleted.