Support .exit

[knpwrs]

I've gotten accustomed to typing .exit in the Node REPL in order to exit. Frankly, I realize this is a bad habit compared to just using ctrl-d (many more keystrokes are required) but I was wondering if it would be possible for jay to support .exit.

[rsify]

not a big fan of the standard repl's dot commands tbh - i'm thinking of adding this indirectly via some kind of a plugin system in the future, but i'm not exactly sure when that'll happen.

for now though, if you really want the command you can plop the appropriate code here:
https://github.com/nikersify/jay/blob/bf4482942c5836e1200c0158ccf926121e06d8be/source/cli.ts#L208-L211