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.
The buffer now emits a new event 'update-anchors-after-change' to signal that all the anchors have been updated, which is an appropriate time to merge cursors.
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.
Pull out a SnippetExpansion class that is created when a snippet is successfully expanded. This is the only object that gets associated with the edit session.
Snippet placeholders are managed by adding an "anchor range" to the edit session. An anchor range basically tracks two anchors for the start and the end of the range.
Cursors and selections no longer need to handle buffer/display-buffer changes directly. They register anchors with the EditSession, and subscribe to changes in their position instead. This opens up the anchor facility for use by extensions that want to track the screen position of a point in the buffer as the buffer and display buffer change.
