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/vim9_super_this_keywords2.vim

@@ -0,0 +1,63 @@
+vim9script
+
+# VIM_TEST_SETUP hi link vim9Super Todo
+# VIM_TEST_SETUP hi link vim9This Todo
+# See: https://github.com/vim/vim/pull/16476#issuecomment-2638191110
+
+
+class A
+  var _value: any
+
+  def new(value: any)
+    this._BaseInit(value)
+  enddef
+
+  def _BaseInit(value: any)
+    this._value = value
+  enddef
+
+  def Value(): any
+    return this._value
+  enddef
+endclass
+
+class B extends A
+  def new(value: number)
+    super._BaseInit(value)
+  enddef
+
+  def Value(): number
+    echo this
+    return super._value
+  enddef
+endclass
+
+class C extends A
+  #### E117
+  #   def new(value: string)
+  #       super(value)
+  #   enddef
+
+  #### E1034
+  #   def new(super._value)
+  #   enddef
+
+  #### E1034
+  #   def new(value: string)
+  #       super._value = value
+  #   enddef
+
+  #### E1356
+  #   def Super(): A
+  #       return super
+  #   enddef
+
+  def This(): C
+    return this
+  enddef
+endclass
+
+echo 1 == A.new(1).Value()
+echo 2 == B.new(2).Value()
+defcompile C
+