Handle default index commands similarly to other @-commands

* tp/Texinfo/ParserNonXS.pm (parser, simple_parser),
tp/Texinfo/XS/parsetexi/indices.c (struct def, default_indices)
(init_index_commands), tp/Texinfo/command_data.txt,
tp/maintain/regenerate_commands_perl_info.pl: add default index
commands to tp/Texinfo/command_data.txt like other builtin @-commands.
6 files changed