Improve REPL's multiline parsing

[bugarela]

For now, the REPL only supports parsing multiline things when there is some sort of open braces that still need to be closed when a newline appears. We should take approaches from other languages and try to make this better.

@josef-widder was trying to paste runs like this in the REPL, and the only way to do it right now is to add some curly braces around it:

init
  .then(something)
  .then(something_else)