css.lang: Add container contexts to help lang file organization
css.lang: Add container contexts to help lang file organization

This adds:

* all-* contexts to make referencing all contexts of a particular group
(e.g. basic data types) easier
* declaration context to make matching style declarations reusable
* comment and string convenience contexts
* comma, colon and semicolon contexts to replace punctuators (and
actually match these symbols and block delimiters in all contexts)

https://bugzilla.gnome.org/show_bug.cgi?id=796130

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview