In ST2 I could browse the defualt packages for each language, open the -tmLanguage file for the language definitions and edit it.
(to add a new file extension to the Listed file types , for example)
in ST3 browse packages sends me to C:\Users\<user>\AppData\Roaming\Sublime Text 3\Packages
and I only see and edit teh few packages I have installed\
default packages appear to be compiled binaries in C:\Program Files\Sublime Text 3\Packages