Support Psysh instead of Boris

[Psysh](https://github.com/bobthecow/psysh) vs. [Boris](https://github.com/d11wtq/boris):

* dependency on ext-pcntl is optional
* doesn't need trailing semicolon to finish commands
* supports the `$_` variable

Disadvantages:

* has dependency on symfony-console
* doesn't support the `$in_a_long_running_operation` trick

Overall, I think it's a better fit as an enhanced `wp shell`. Just need to see how easy it is to integrate.