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_lambda.vim

@@ -0,0 +1,116 @@
+vim9script
+
+# Vim 9 lambda expressions
+
+var Foo: func
+var expr = 0
+
+# without return type
+Foo = () => expr
+Foo = (_) => expr
+Foo = (x) => expr
+
+Foo = (...y) => expr
+Foo = (_, ...y) => expr
+Foo = (x, ...y) => expr
+
+Foo = (x, y) => expr
+
+Foo = (_: number) => expr
+Foo = (x: number) => expr
+
+Foo = (...y: list<number>) => expr
+Foo = (_: number, ...y: list<number>) => expr
+Foo = (x: number, ...y: list<number>) => expr
+
+Foo = (x: number, y: number) => expr
+
+# with return type
+Foo = (): number => expr
+Foo = (_): number => expr
+Foo = (x): number => expr
+
+Foo = (...y): number => expr
+Foo = (_, ...y): number => expr
+Foo = (x, ...y): number => expr
+
+Foo = (x, y): number => expr
+
+Foo = (_: number): number => expr
+Foo = (x: number): number => expr
+
+Foo = (...y: list<number>): number => expr
+Foo = (_: number, ...y: list<number>): number => expr
+Foo = (x: number, ...y: list<number>): number => expr
+
+Foo = (x: number, y: number): number => expr
+
+
+# post operator comments
+
+Foo = () => # comment
+  expr
+Foo = () =>
+  # comment
+  expr
+Foo = () =>
+
+  # comment
+
+  expr
+
+
+# line continuations
+
+Foo = (x: string,
+      \ y: number,
+      \ z: bool): number => expr
+# FIXME
+Foo = (x: string,
+      \ y: number,
+      \ z: bool): number
+      \ => expr
+
+
+# funcref call
+
+echo (() => 42)()
+echo ((x: string): number => 42)("foo")
+
+
+# :help vim9-lambda
+
+var list = [1, 2, 3]
+echo filter(list, (k, v) =>
+		v > 0)
+echo filter(list, (k,
+      \	v)
+      \	=> v > 0)
+
+var Callback = (..._) => 'anything'
+echo Callback(1, 2, 3)  # displays "anything"
+
+var Lambda = (arg) => {
+	g:was_called = 'yes'
+	return expr
+    }
+
+var count = 0
+var timer = timer_start(500, (_) => {
+	 count += 1
+	 echom 'Handler called ' .. count
+     }, {repeat: 3})
+
+var dict = {}
+var d = mapnew(dict, (k, v): string => {
+     return 'value'
+   })
+
+
+# Issue #15970 (vim9: Restore and extend the recognition of Enum body items)
+
+def Op(): func(func(number, number): number): func(number, Digit): number
+    return (F: func(number, number): number) =>
+	(x: number, y: Digit): number => F(x, y.value)
+enddef ####################### ^ vimCommand?
+