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_menu_04.dump

@@ -12,7 +12,7 @@
 |"+0#0000e05&| |a| |m|e|n|u| |i|t|e|m| |n|a|m|e| |c|a|n@1|o|t| |s|t|a|r|t| |w|i|t|h| |'|.|'| +0#0000000&@34
 @75
 |d+0#af5f00255&|e|f| +0#0000000&|H|i|s|t|o|r|y|J|u|m|p|M|e|n|u|(+0#e000e06&|)| +0#0000000&@53
-@4|p+0#af5f00255&|o|p|u|p|.|F+0#0000000&|i|l|t|e|r|M|e|n|u|(+0#e000e06&|"+0#e000002&|J|u|m|p| |h|i|s|t|o|r|y|"|,+0#0000000&| |d+0#00e0e07&|i|r|_|h|i|s|t|,+0#0000000&| @28
+@3| +0#00e0e07&|p|o|p|u|p|.+0#af5f00255&|F+0#0000000&|i|l|t|e|r|M|e|n|u|(+0#e000e06&|"+0#e000002&|J|u|m|p| |h|i|s|t|o|r|y|"|,+0#0000000&| |d+0#00e0e07&|i|r|_|h|i|s|t|,+0#0000000&| @28
 @8|(+0#e000e06&|r+0#00e0e07&|e|s|,+0#0000000&| |_+0#00e0e07&|)+0#e000e06&| +0#0000000&|=+0#af5f00255&|>| +0#0000000&|{+0#e000e06&| +0#0000000&@53
 @12|H|i|s|t|o|r|y|J|u|m|p|(+0#e000e06&|r+0#00e0e07&|e|s|.+0#af5f00255&|t+0#00e0e07&|e|x|t|)+0#e000e06&| +0#0000000&@41
 @8|}+0#e000e06&|)| +0#0000000&@64