Remove ASTNode.prototype._append

Instead of exposing a private version of the
append method, just expose enumerated types
that are for internal use only.

In other words, special append behavior is
only invoked for those special node types.

TODO: Hide private enums from the outside
world. These are:

- TH_TMP
- TD_TMP
- TR_TMP
- UL_LI
- OL_LI
- CLASS
- ID