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_command_03.dump

@@ -2,7 +2,7 @@
 @75
 |"+0#0000e05&| |e|r@1|o|r|s| +0#0000000&@66
 @75
-|c+0#af5f00255&|o|m@1|a|n|d|!| +0#0000000&|-+0#ffffff16#ff404010|b|a|d|a|t@1|r|=+0#af5f00255#ffffff0|a+0#0000000&|r|g|u|m|e|n|t|s| |-+0#e000e06&|b|a|n|g| +0#0000000&|-+0#ffffff16#ff404010|b|a|d|a|t@1|r| +0#0000000#ffffff0|-+0#e000e06&|n|a|r|g|s|=|*+0#00e0003&| +0#0000000&|F|o@1| |e+0#af5f00255&|c|h|o| +0#0000000&|"+0#e000002&|F|o@1|"| +0#0000000&@8
+|c+0#af5f00255&|o|m@1|a|n|d|!| +0#0000000&|-+0#ffffff16#ff404010|b|a|d|a|t@1|r|=+0#af5f00255#ffffff0|a+0#00e0e07&|r|g|u|m|e|n|t|s| +0#0000000&|-+0#e000e06&|b|a|n|g| +0#0000000&|-+0#ffffff16#ff404010|b|a|d|a|t@1|r| +0#0000000#ffffff0|-+0#e000e06&|n|a|r|g|s|=|*+0#00e0003&| +0#0000000&|F|o@1| |e+0#af5f00255&|c|h|o| +0#0000000&|"+0#e000002&|F|o@1|"| +0#0000000&@8
 > @74
 |"+0#0000e05&| |d|e|l|e|t|e| +0#0000000&@66
 @75