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_ex_function_09.dump

@@ -8,7 +8,7 @@
 @75
 |"+0#0000e05&| |I|s@1|u|e| |#|1|6|2|4|3| |(|v|i|m|s|c|r|i|p|t| |d|e|f| |p|a|r|a|m|e|t|e|r|s| |s|y|n|t|a|x| |h|i|g|h|l|i|g|h|t| |i|s| |w|r|o|n|g|)| +0#0000000&@7
 @75
-|f+0#af5f00255&|u|n|c|t|i|o|n| +0#0000000&|T|e|s|t|(+0#e000e06&|l+0#00e0e07&|i|n|e|s| +0#0000000&|=+0#af5f00255&| +0#0000000&|[|l+0#00e0e07&|i|n|e|(+0#e000e06&|'+0#e000002&|.|'|)+0#e000e06&|,+0#0000000&| |l+0#00e0e07&|i|n|e|(+0#e000e06&|'+0#e000002&|.|'|)+0#e000e06&|]+0#0000000&|)+0#e000e06&| +0#0000000&@29
+|f+0#af5f00255&|u|n|c|t|i|o|n| +0#0000000&|T|e|s|t|(+0#e000e06&|l+0#00e0e07&|i|n|e|s| +0#0000000&|=+0#af5f00255&| +0#0000000&|[+0#e000e06&|l+0#00e0e07&|i|n|e|(+0#e000e06&|'+0#e000002&|.|'|)+0#e000e06&|,+0#0000000&| |l+0#00e0e07&|i|n|e|(+0#e000e06&|'+0#e000002&|.|'|)+0#e000e06&|]|)| +0#0000000&@29
 |e+0#af5f00255&|n|d|f|u|n|c|t|i|o|n| +0#0000000&@63
 @75
 @75