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_expr_13.dump

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