Having the @newline instance variable in every node adds up, and
since it is so rarely used, we only want to add it when necessary.

Moving this into an autoloaded file and moving the instance variable
out of the default initializers reduces allocated memory because the
nodes are now smaller and some fit into the compact list. On my
machine, I'm seeing about an 8% drop.

https://github.com/ruby/prism/commit/eea92c07d2