WIP: giving suggestions when unexpected indent happens

[timo]

when it can parse the beginning of a list item or map item after a map but with too much indentation, it will panic. on the way "out" it will register all possible indents it's backtracked over, so that we can give a list of suggestions for nested things

this may also want to differentiate between indents registered for maps and for lists, so that it doesn't suggest to indent a list entry with the same amount as a map key, because that'd still be wrong.

i'm not actually suggesting this to be pulled right away, i'd like to have a discussion first :)