Adds indentation nodes (:indent and :indent_one_less) where appropriate.

Surface.Formatter.Render.node/2 is responsible for adding the appropriate level of indentation. It keeps track of the indentation level based on how "nested" a node is. While running Formatter Phases, it's not necessary to keep track of that detail.

:indent_one_less exists to notate the indentation that should occur before a closing tag, which is one less than its children.

Relies on Newlines phase, which collapses :newline nodes to at most 2 in a row.