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_let_heredoc_10.dump

@@ -16,5 +16,5 @@
 ||+0#0000e05#a8a8a8255| |A+0#e000e06#ffffff0|!|@|#|$|%|^|&|*|(|)|_|+| +0#0000000&@59
 | +0#0000e05#a8a8a8255@1| +0#0000000#ffffff0@72
 | +0#0000e05#a8a8a8255@1|"+0&#ffffff0| |e|r@1|o|r| |-| |l|e|a|d|i|n|g| |l|o|w|e|r|c|a|s|e| |c|h|a|r|a|c|t|e|r| +0#0000000&@35
-| +0#0000e05#a8a8a8255@1|l+0#af5f00255#ffffff0|e|t| +0#0000000&|f+0#00e0e07&|o@1| +0#0000000&|=+0#af5f00255&|<@1| +0#0000000&|a+0#af5f00255&|!|@+0#0000000&|#|$|%+0#af5f00255&|^+0#0000000&|&|*+0#af5f00255&|(+0#e000e06&|)|_+0#0000000&|++0#af5f00255&| +0#0000000&@47
+| +0#0000e05#a8a8a8255@1|l+0#af5f00255#ffffff0|e|t| +0#0000000&|f+0#00e0e07&|o@1| +0#0000000&|=+0#af5f00255&|<@1| +0#0000000&|a+0#00e0e07&|!+0#0000000&|@|#|$|%+0#af5f00255&|^+0#0000000&|&|*+0#af5f00255&|(+0#e000e06&|)|_+0#0000000&|++0#af5f00255&| +0#0000000&@47
 @57|1|7|9|,|1| @8|9|7|%|