Add new ligatures

Thanks to @Jakob6174 we now have some new ligatures.

// new ligatures v2.5

- colon_colon                         ::
- exclam_exclam                       !!
- semicolon_semicolon                 ;;
- less_less                           <<
- greater_greater                     >>
- hyphen_equal (minus_equal)          -=
- asterisk_equal                      *=
- slash_equal                         /=
- less_slash_greater                  </>
- less_less_equal                     <<=
- quotedbl_quotedbl_quotedbl          """
- quotesingle_quotesingle_quotesingle '''

See README and Ligature Preview