Add pbcopy

This will copy any string to my macOS clipboard:

```rb
some_large_sql_string.pbcopy
```

It works by creating a new output stream that pipes directly into the
`pbcopy` command using `IO.popen`. `popen` stands for "pipe open" and is
similar to the standard C `popen(3)` function. Imagine it's just like
`$stdout` but you can attach a command to it really easily.

It invokes `popen` with a block and simply `puts` the string into it.
Using a block argument automatically closes the IO.

Big shoutout to the Avdi Grimm and his https://graceful.dev/ videos for
teaching me about open pipe (which looks like `open("|command")`
instead), and postmodern for teaching me that `popen` is the better
choice because it shows more intention than `open("|command")`:
https://bugs.ruby-lang.org/issues/19630