Fix handling of opener for nested text objects

[ascandella]

Prior to this commit, we would naively search for the previous e.g. "{"
character when you wanted to "ci{".

However, if you were inside two levels of nesting:

  one {
    two {}
    // cursor here
  }

Then the backwards search would take you to the wrong place.

Switching to the same logic for forward searching, we now handle this
case properly.

[jpoon]

Awesome! Thanks @sectioneight 🚀