This is everything that `irb` uses. It works in their test-suite, but there are 20 failures when using the shim that I haven't looked into at all.
`parse` is not used by `irb`. `scan` is, and it's basically `parse` but also including errors. `irb` doesn't seem to care about the errors, so I didn't implement that.
https://github.com/ruby/prism/commit/2c5826b39f
15939e3c30