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_def_nested_00.dump

@@ -1,5 +1,6 @@
 >v+0#af5f00255#ffffff0|i|m|9|s|c|r|i|p|t| +0#0000000&@64
 |#+0#0000e05&| |V|i|m|9| |:|d|e|f| |c|o|m@1|a|n|d| |(|n|e|s|t|e|d|)| +0#0000000&@46
+|#+0#0000e05&| |V|I|M|_|T|E|S|T|_|S|E|T|U|P| |h|i| |l|i|n|k| |v|i|m|9|T|h|i|s| |T|o|d|o| +0#0000000&@36
 @75
 |c+0#af5f00255&|l|a|s@1| +0#0000000&|T|e|s|t| @64
 @4|c+0#af5f00255&|o|n|s|t| +0#0000000&|n+0#00e0e07&|a|m|e|:+0#0000000&| |s+0#00e0003&|t|r|i|n|g| +0#0000000&@52
@@ -13,8 +14,7 @@
 @12|r+0#af5f00255&|e|t|u|r|n| +0#0000000&|G|i|v|e|N|a|m|e|(+0#e000e06&|)| +0#0000000&@45
 @8|e+0#af5f00255&|n|d@1|e|f| +0#0000000&@60
 @75
-@8|t+0#00e0e07&|h|i|s|.+0#af5f00255&|n+0#0000000&|a|m|e| |=+0#af5f00255&| +0#0000000&|N|a|m|e|(+0#e000e06&|)| +0#0000000&@48
+| +0#00e0e07&@7|t+0#0000001#ffff4012|h|i|s|.+0#af5f00255#ffffff0|n+0#00e0e07&|a|m|e| +0#0000000&|=+0#af5f00255&| +0#0000000&|N|a|m|e|(+0#e000e06&|)| +0#0000000&@48
 @4|e+0#af5f00255&|n|d@1|e|f| +0#0000000&@64
 |e+0#af5f00255&|n|d|c|l|a|s@1| +0#0000000&@66
-@75
 @57|1|,|1| @10|T|o|p|