OutdentModifier class

Removes one indent level when Backspace is pressed inside a line's leading whitespace — the "outdent on Backspace" that issue #21 asks for.

The trigger is a deletion rather than a typed character, so this registers as a deletion modifier: see CodeModifier.deletesChar. It keys on the space the backspace removes, which is what a caret inside the leading whitespace always sits behind. Without that registration no modifier could ever fire on a backspace, which is the gap issue #21 reports.

Outside the leading whitespace it returns null and the ordinary one-character delete stands, because a backspace that removes real code should remove one character rather than reindent the line.

Its semantics are deliberately narrower than CodeController.outdentSelection's, the Shift+Tab command: that command also strips tabs and trims whatever indent is left when it is not a multiple of tabSpaces (trimLeft), while this modifier declines over a tab and empties a partial indent exactly (clamp) — the Backspace gesture stays conservative where the command is free to reindent.