This is still in progress. You can't *redo* snippet expansion and restore tab stops. Also, this commit performs all changes associated with snippet expansion in a transaction.
The `do`, `undo`, and `redo` methods on operations take an optional editSession argument, which can be used to determine the context in which they are being run. We restore selections on that edit session instead of the session where the operations originally occurred.
The `transact` method takes a function and batches all operations within that function as a single transaction to be undone and redone. The edit session now uses generic operations to restore selection state around transactions. The `undo` and `do` methods on operations are now optional. In addition, the undo manager now supports an optional `redo` method on an operation for code that should *only* be run on redo, and not when the operation is initially pushed. This is used by edit session to restore selection state after redo.
