Specify `indent`

node.position.indent isn’t used a lot, but it could be specced better.

Currently, it’s a list of integers, if a node spans multiple lines, where each value refers to the column a line (node.position.start.line + index).
This only supports the start of a line (which is useful in markdown). But not the end of a line.
It’s also awkward to access, as there’s no explicit line access.

It could make sense for indent to be an Array.<{start: Point, end: Point}>.