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/input/vim_lambda.vim

@@ -0,0 +1,120 @@
+" Vim lambda expressions
+
+
+let expr = 42
+
+let Foo = {-> expr}
+let Foo = {_ -> expr}
+let Foo = {... -> expr}
+let Foo = {x -> expr}
+let Foo = {x, _ -> expr}
+let Foo = {x, ... -> expr}
+let Foo = {x, y -> expr}
+
+
+" line continuations
+
+let Foo = {->
+      "\ comment
+      \ expr
+      \ }
+let Foo = {_ ->
+      "\ comment
+      \ expr
+      \ }
+let Foo = {... ->
+      "\ comment
+      \ expr
+      \ }
+let Foo = {x ->
+      \ expr
+      "\ comment
+      \ }
+let Foo = {x, y ->
+      "\ comment
+      \ expr
+      \ }
+
+let Foo = {
+      \ ->
+      "\ comment
+      \ expr
+      \ }
+let Foo = {x
+      \ ->
+      "\ comment
+      \ expr
+      \ }
+let Foo = {x, y
+      \ ->
+      "\ comment
+      \ expr
+      \ }
+
+let Foo = {x,
+      \ y,
+      \ z -> expr}
+
+let Foo = {
+      \ x,
+      \ y,
+      \ z
+      \ ->
+      "\ comment
+      \ expr
+      \ }
+
+let Foo = {-> [
+      \ 42,
+      \ 83
+      \]}
+
+let Foo = {-> {
+      \ 'a': 42,
+      \ 'b': 83
+      \}}
+
+let Foo = {-> #{
+      \ a: 42,
+      \ b: 83
+      \}}
+
+let Foo = {-> {->[
+      \ 42,
+      \ 83
+      \]}}
+
+let Foo = {-> {-> {
+      \ 'a': 42,
+      \ 'b': 83
+      \}}}
+
+let Foo = {-> {-> #{
+      \ a: 42,
+      \ b: 83
+      \}}}
+
+" :help lambda
+
+:let F = {arg1, arg2 -> arg1 - arg2}
+:echo F(5, 2)
+
+:let F = {-> 'error function'}
+:echo F('ignored')
+
+:function Foo(arg)
+:  let i = 3
+:  return {x -> x + i - a:arg}
+:endfunction
+:let Bar = Foo(4)
+:echo Bar(6)
+
+:echo map([1, 2, 3], {idx, val -> val + 1})
+" [2, 3, 4]  
+
+:echo sort([3,7,2,1,4], {a, b -> a - b})
+" [1, 2, 3, 4, 7]
+:let timer = timer_start(500,
+		\ {-> execute("echo 'Handler called'", "")},
+		\ {'repeat': 3})
+