runtime(vim): Update base-syntax, match Vim9 boolean and null literals in parens

- Match Vim9 boolean and null literals in parenthesised expressions and
  function argument lists.
- Match read-only registers in expressions.

closes: #16622

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/vim9_expr.vim

@@ -30,6 +30,7 @@ echo g:foo ? g:foo : g:bar
 echo $FOO ? $FOO : $BAR
 echo True() ? True() : False()
 echo @a ? @a : @b
+echo (true) ? (true) : (false)
 echo (1) ? (1) : (0)
 
 # Falsy
@@ -52,9 +53,15 @@ echo g:foo ?? g:foo
 echo $FOO ?? $FOO
 echo True() ?? True()
 echo @a ?? @a
+echo (true) ?? (true)
 echo (1) ?? (1)
 
 
+# Function calls
+
+Foo(true, false, null)
+
+
 # Command {expr} arguments
 
 if true