The name gtk_text_*_begins_* was used only for begins_tag(). All other
similar functions use "starts": starts_line(), starts_word(), etc.

So for consistency, add gtk_text_iter_starts_tag() and deprecate
gtk_text_iter_begins_tag().

Also change (allow-none) to (nullable), to use the new annotation.

https://bugzilla.gnome.org/show_bug.cgi?id=759092