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_keywords.vim

@@ -0,0 +1,87 @@
+vim9script
+
+# Vim9 this and super keywords
+# VIM_TEST_SETUP hi link vim9This Todo
+# VIM_TEST_SETUP hi link vim9Super Todo
+
+def Echo(...args: list<any>)
+  echo args
+enddef
+
+class Foo
+  var x: number = 42
+  var y: number = this.x + 41
+  var z: number = this.x + this.y
+
+  def new()
+    echo this.x this.y this.z
+  enddef
+
+  def newXY(this.x, this.y, this.z)
+  enddef
+
+  def Def1(arg = this.x)
+    this.y = arg
+    this.z += arg
+  enddef
+
+  def Def2(arg = (this.x + this.y + this.z))
+    Echo(this, this.x, this.y, this.z)
+    this->Echo(this.x, this.y, this.z)
+  enddef
+
+  def Def3(): Foo
+    return this
+  enddef
+
+  def Def4(arg: Foo = this): Foo
+    return arg
+  enddef
+endclass
+
+class Bar extends Foo
+  def Def1()
+    super.Def1()
+  enddef
+
+  def Def2()
+    var a = super.x * super.y * super.z
+    var b = [super.x, super.y, super.z]
+    var c = {super: super.x, this: super.y, true: super.z}
+    var d = {super: c, this: c}
+    echo c.super
+    echo c.this
+    echo d.super.this
+    echo d.this.super
+    echo a b c
+  enddef
+
+  def Def5()
+    var a = this.x * this.y
+    var b = (this.x * this.y)
+    var c = [this.x, this.y]
+    var d = {super: this.x, this: this.y}
+    echo a b c d
+  enddef
+
+  def Def6()
+    var x = this#super#x
+    var y = super#this#y
+    this#super#Func()
+    super#this#Func()
+  enddef
+
+  def Def7(arg = super.Def3())
+    echo arg
+  enddef
+
+  def Def8(): number
+    var F = () => this.x
+    var G = () => super.x
+    return F() + G()
+  enddef
+endclass
+
+defcompile Foo
+defcompile Bar
+