runtime(vim): Update syntax, improve user-command matching

- Match -addr and -keepscript attributes and generate -addr values.
- Match attribute errors where = is specified.
- Highlight attributes with Special like other Ex command options.
- Don't highlight user-specified completion function args.
- Match :delcommand -buffer attribute.

closes: #15586

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim_ex_command.vim

@@ -1,5 +1,74 @@
-" Vim :command command
+" Vim :command, :delcommand and :comclear commands
+
+
+" list
+
+command
+command F
+
+" define
+
+command  Foo echo "Foo"
+command! Foo echo "Foo"
+command! Foo echo "Foo" | echo "Bar"
+
+command! Foo {
+  echo "Foo"
+  echo "Bar"
+  echo "Baz"
+}
+
+command! -addr=arguments -bang -bar -buffer -complete=arglist -count=1 -keepscript -nargs=* -range=% -register Foo
+      \ echo "Foo"
+
+command! -complete=custom,Completer1 Foo echo "Foo"
+command! -complete=customlist,Completer2 Foo echo "Foo"
+
+function Foo()
+  command! Foo echo "Foo (defined in :function)"
+endfunction
+
+def Foo2()
+  command! Foo echo "Foo (defined in :def)"
+enddef
+
+" multiline define
+
+" command!
+"       \ -addr=lines
+"       \ -bang
+"       \ -bar
+"       \ -buffer
+"       \ -complete=buffer
+"       \ -count
+"       \ -nargs=*
+"       \ -range
+"       \ -register
+"       \ -keepscript
+"       \ Foo 
+"       \ echo "FOO"
+
+" errors
+
+command! -badattr=arguments -bang -badattr -nargs=* Foo echo "Foo"
+
+" delete
+
+delcommand Foo
+delcommand -buffer Foo
+
+delcommand Foo | echo "..."
+delcommand -buffer Foo | echo "..."
+
+delcommand Foo " comment
+delcommand -buffer Foo " comment
+
+comclear
+comclear " comment
+comclear | echo "..."
+
 
 " Issue #14135
 
 com Foo call system('ls')
+