runtime(vim): Update base-syntax, improve variable matching

- Match variables after operators, including line continuations.
- Match option variables without leading whitespace.
- Explicitly match expression subscripts.
- Match Vim9 variables in LHS of assignments and method calls.
- Match option variables (&option) with a dedicated syntax group like
  environment variables.
- Match list literals, fixes: #5830
- Match :{un}lockvar arguments.
- Match registers and environment variables in :let unpack lists.
- Match lambda expressions
- Match Vim9 scope blocks
- Match variables in :for subject
- Highlight user variables with Normal
- Improve this/super keyword matching, fixes: #15970

closes: #16476

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

runtime/syntax/testdir/input/vim_ex_for.vim

@@ -0,0 +1,60 @@
+" Vim :for command
+
+
+" :for {var} in {object}
+
+let expr = [42]
+
+for foo in expr
+  echo foo
+endfor
+
+for foo in expr " comment
+  echo foo
+endfor
+
+for foo in
+      "\ comment
+      \ expr
+  echo foo
+endfor
+
+for foo in expr | echo foo | endfor
+
+for foo in [42]
+  echo foo
+endfor
+
+for foo in [42] | echo foo | endfor
+
+echo "foo" | for foo in expr
+  echo foo
+endfor
+
+
+" :for [{var1}, {var2}, ...] in {listlist}
+
+let expr = [[42, 83]]
+
+for [foo, bar] in expr
+  echo foo bar
+endfor
+
+for [foo, bar] in expr " comment
+  echo foo bar
+endfor
+
+for [foo, bar] in
+      "\ comment
+      \ expr
+  echo foo bar
+endfor
+
+for [foo, bar] in expr | echo foo bar | endfor
+
+for [foo, bar] in [[42, 83]]
+  echo foo bar
+endfor
+
+for [foo, bar] in [[42, 83]] | echo foo bar | endfor
+