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/dumps/vim_expr_01.dump

@@ -17,4 +17,4 @@
 |e+0#af5f00255&|c|h|o| +0#0000000&|'+0#e000002&|f|o@1|'+0#e000e06&@1|'+0#e000002&| +0#0000000&@62
 |e+0#af5f00255&|c|h|o| +0#0000000&|'+0#e000002&|f|o@1|'+0#e000e06&@1|b+0#e000002&|a|r|'| +0#0000000&@59
 @75
-@57|1|5|,|1| @10|6|%| 
+@57|1|5|,|1| @10|5|%|