If the selected entry is the last entry in a directory that *itself* is the last entry of its parent, we need to search upward through our ancestors for the first directory that has a following sibling entry. Also, if the directory is empty, the attempt to select its first element will fail, so we need to search for a next sibling as well.
LanguageMode is the central point for all language-specific behavior associated with an EditSession. There is one LanguageMode instance per EditSession. LanguageMode has access to the EditSession and its TokenizedBuffer, and in reverse the EditSession, DisplayBuffer, and TokenizedBuffer also make use of LanguageMode. This is a bit incestuous, but I think it's okay because you can think of LanguageMode as a swappable strategy object that governs language-specific aspects of that constellation of objects.
Operations now return a range to select rather than actually performing a selection in `execute`. This allows the composite command to aggregate all ranges to select and only change the selection if it's non empty. Before we had to clear the selections ahead of time and then rely on each operation to imperatively add its selection. This is easier to control. It also ensures that when we execute a previewed operation, we don't add a selection but instead change the selection.
Two main changes: First, use absolute position left/right/top/bottom of 0 as a hack to achieve 100% width and height of the flexbox that contains the gutter and scroll view when it is inside an editor that is itself a flexbox item. Now we don't rely on the editor having a definite height because of the absolute positioning. This seemed to confuse the newer flexbox implementation, so I fell back to the old one which seems to work better than last time I tried it for this purpose (it had rendering artifacts before)
If the stdout callback is fired with partial data, and it ends right after a path, we need to ensure we don't handle it as if there's an extra newline.
