syntax: texinfo: colorize the special @-plus-punctuation commands too

Also, allow an @} and another @command{} within a pair of braces,
colorize only the valid @-commands with uppercase in their names,
unbold enclosed command arguments, colorize the directory entries
for the manual, and properly colorize also the comments that use
the full @comment command.

Reference:
  https://www.gnu.org/software/texinfo/manual/texinfo/html_node/Command-List.html

Testing was done with:
  info texinfo @-C "Command List" --output=list
  sed -i -e "s/^'//" -e "s/'$//" list
  nano list -Ytexinfo

syntax/texinfo.nanorc

@@ -8,17 +8,24 @@ header "^\\input texinfo"
 magic "Texinfo source"
 comment "@c "
 
+# How the manual gets listed in the directory node.
+color purple start="^@direntry" end="^@end direntry"
+
 # Command arguments, trailing and enclosed.
 color cyan "^@[a-z]+[[:space:]]+.*"
-color brightmagenta "@[a-zA-Z]+\{[^}]*\}"
+color lightmagenta "@([a-z]+|,|H|U)\{([^}]|@\}|@[a-z]+\{[^}]*\})*\}"
 # Commands themselves.
-color yellow "@[a-zA-Z]+\{?|\}"
+color yellow "@([a-z]+\{?|[,HU]\{|(AA|AE|DH|L|OE?|(La)?TeX|TH)\{)|\}"
+color pink "@[!"'&*./:=?@\^`{}~-]"
+
+# Special separator for headings and footings.
+color mint "@\|"
 
 # Menu items.
 color brightred "^\*[[:space:]]+.*::.*"
 
 # Comments.
-color green "@c[[:space:]]+.*"
+color green "@c(omment)?[[:space:]]+.*"
 
 # Trailing whitespace.
 color ,green "[[:space:]]+$"