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_line_continuation_02.dump

@@ -8,13 +8,13 @@
 @6|\+0#e000e06&| +0#0000000&|c+0#00e0e07&|:+0#0000000&| |3+0#e000002&| +0#0000000&@62
 @6|\+0#e000e06&|}| +0#0000000&@66
 @75
-|l+0#af5f00255&|e|t| +0#0000000&|a+0#00e0e07&|r@1|a|y| +0#0000000&|=+0#af5f00255&| +0#0000000&|[| @61
+|l+0#af5f00255&|e|t| +0#0000000&|a+0#00e0e07&|r@1|a|y| +0#0000000&|=+0#af5f00255&| +0#0000000&|[+0#e000e06&| +0#0000000&@61
 @6|"+0#0000e05&|\| |e|l|e|m|e|n|t| |1| +0#0000000&@56
 @6|\+0#e000e06&| +0#0000000&|1+0#e000002&|,+0#0000000&| @64
 @6|"+0#0000e05&|\| |e|l|e|m|e|n|t| |2| +0#0000000&@56
 @6|\+0#e000e06&| +0#0000000&|2+0#e000002&|,+0#0000000&| @64
 @6|"+0#0000e05&|\| |e|l|e|m|e|n|t| |3| +0#0000000&@56
 @6|\+0#e000e06&| +0#0000000&|3+0#e000002&| +0#0000000&@65
-@6|\+0#e000e06&|]+0#0000000&| @66
+@6|\+0#e000e06&|]| +0#0000000&@66
 |~+0#4040ff13&| @73
 | +0#0000000&@56|3|7|,|7| @9|B|o|t|