Add new file extensions (see description)

This commit adds three file extensions that Textual is built to
respond. These file extensions allow Textual to install certain addons
easily.

.TxtlScript is an AppleScript file. It replaces .scpt as the standard
script extension used by Textual. When opened, the user is presented
with a save dialog which brings them to the "Application Scripts"
folder owned by Textual. Since Textual cannot install there without
user expclictly selecting the folder, we bring them right to it. They
just have to click save.

.TxtlBundle is the new file extension for extensions. Replaces .bundle.
This file extension currently does nothing other than load like the
previous file extension.

.TxtlStyle will be the file extension used by Style packages. It
currently does nothing.