Support xxd's -g flag

xxd has -g flag:

Usage:
       xxd [options] [infile [outfile]]
    or
       xxd -r [-s [-]offset] [-c cols] [-ps] [infile [outfile]]
Options:
    -a          toggle autoskip: A single '*' replaces nul-lines. Default off.
    -b          binary digit dump (incompatible with -ps,-i,-r). Default hex.
    -C          capitalize variable names in C include file style (-i).
    -c cols     format <cols> octets per line. Default 16 (-i: 12, -ps: 30).
    -E          show characters in EBCDIC. Default ASCII.
    -e          little-endian dump (incompatible with -ps,-i,-r).
    -g          number of octets per group in normal output. Default 2 (-e: 4).
    -h          print this summary.
    -i          output in C include file style.
    -l len      stop after <len> octets.
    -o off      add <off> to the displayed file position.
    -ps         output in postscript plain hexdump style.
    -r          reverse operation: convert (or patch) hexdump into binary.
    -r -s off   revert with <off> added to file positions found in hexdump.
    -s [+][-]seek  start at <seek> bytes abs. (or +: rel.) infile offset.
    -u          use upper case hex letters.
    -v          show version: "xxd V1.10 27oct98 by Juergen Weigert".

For example, xxd -l 32 ftplugin/xxd/cursor.vim:

00000000: 6675 6e63 7469 6f6e 2120 733a 6869 6768  function! s:high
00000010: 6c69 6768 7428 290a 2020 6966 2065 7869  light().  if exi

and xxd -g 4 -l 32 ftplugin/xxd/cursor.vim:

00000000: 66756e63 74696f6e 2120733a 68696768  function! s:high
00000010: 6c696768 7428290a 20206966 20657869  light().  if exi

When the cursor is on the first 66, the highlighting goes to the fifth character, t.

Start with ftplugin/xxd/cursor.vim and inspect how the cursor position is derived from normal xxd output. Compare the shown default grouping with the xxd -g 4 example, then verify that placing the cursor on the first 66 highlights the fifth character as described. No test file is mentioned, so use these command-output examples as the completion check.