Toggle list type sections

[bantic]

Add postEditor#toggleSection, postEditor#setRange, editor#selectRange APIs

toggleSection is list-section and list-item aware and will toggle a markup section to/from a list item. Toggling a markup section that is adjacent to a list will join it with that list. Toggling a list item will turn it into a markup section and separate it from the before/after list section. The postEditor run loop will join contiguous list sections of the same type during the before_complete callback queue. This means that deleting a markup section that was between two lists will also cause the lists to be joined together.

There is some internal refactoring of the cursor movement/selection methods. It is possible that a postEditor method returns a position that has a section property that is no longer part of the post abstract after the postEditor's run() finishes. This can happen when a markup section is turned into a listSection+listItem and then (in the before_complete queue) merged with an adjacent list.
As a result, it is no longer always safe to use a returned Position with a call to one of the cursor movement methods on editor.cursor (e.g. moveToPosition/selectRange/selectSection). The postEditor#setRange method is added to handle this case.
Mutation methods on the postEditor that could destroy/detach a section can now introspect the current range and update it if they are removing/changing a section that is part of the active range. The postEditor#_joinContiguousListSections is the only method that does this so far, but eventually all of the methods should be updated to update an active range when necessary. The postEditor automatically renders the updated range at the end of its run loop if postEditor#setRange was called.

This is ready for review and merge. If this looks good I will update the documentation to reflect the new APIs shortly before/after merging.

Possible breaking changes

• removal of the following methods on cursor:
  • selectSections, moveToSection, moveToPosition. None of these was explicitly marked as public.
• removal of editor#moveToPosition. This was not explicitly public before
• The README mentions editor#moveToSection, but this API did not actually exist (it should have been editor.cursor.moveToSection). After merge I'll fix up this part of the readme

fixes #186

[mixonic]

I think this can happen in the walkMarkerableSections above- the array sectionsToChange seems to not serve any logical function

[bantic]

At one point there was an issue with mutation while walking — a section's next would get reassigned and the walkMarkerableSections callback would skip sections. I will try removing this and see if the tests still pass.

[bantic]

Removed it and tests are still 🍏, seems good

[mixonic]

Was there a bug of note in the linked list implementation?

[bantic]

Was there a bug of note in the linked list implementation?

The heuristic for detecting if an item is already in a list doesn't cover every case. In particular, an item can be the only item in another list, in which case its next and prev will be null, and no error will be thrown. In addition there's a code path that is dead in all circumstances except in this edge case, in which case it (incorrectly) overwrites the tail of the list with the item-from-another-list. This isn't something that we ever encounter in correctly-running code, however, but it does mean the insertion-guard code doesn't protect against this case (and in fact an unusual bug — as described — occurs). I went a little overboard refactoring; I'll separate the LL code into a separate PR for discussion.

[bantic]

@mixonic This is ready for another review when you have a chance

[mixonic]

So eager to change all these cursor.offsets :-D