Add a new `whereami` command to show where you are in a file.

The only method I would have tested is protected, so none of that lol
It's possible to have backtrace entries with no 'class' or 'function'. I got it to happen by wrapping stuff in my `.config/psysh/config.php` file in closures :)

> PHP Notice:  Undefined index: class in .../psysh/src/Psy/Command/WhereamiCommand.php on line 64

Should add `isset()` on these to keep from getting warnings.