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/vim9_legacy_header_00.dump

@@ -2,7 +2,7 @@
 @75
 |i+0#af5f00255&|f| +0#0000000&|!+0#af5f00255&|h+0#00e0e07&|a|s|(+0#e000e06&|'+0#e000002&|v|i|m|9|s|c|r|i|p|t|'|)+0#e000e06&| +0#0000000&@53
 @2|#| |4+0#e000002&|2| +0#0000000&|"+0#0000e05&| |c|o|m@1|e|n|t| +0#0000000&@58
-@2|s+0#af5f00255&|o|u|r|c|e| +0#0000000&|f|o@1|.+0#af5f00255&|v|i|m| +0#0000000&@58
+@2|s+0#af5f00255&|o|u|r|c|e| +0#00e0e07&|f|o@1|.+0#af5f00255&|v+0#00e0e07&|i|m| +0#0000000&@58
 @2|f+0#af5f00255&|i|n|i|s|h| +0#0000000&@66
 |e+0#af5f00255&|n|d|i|f| +0#0000000&@69
 @75