runtime(vim): Update base-syntax, match Vim9 bool/null literal args to :if/:while/:return

Match Vim9 boolean and null literals in expression arguments of :if,
:elseif, :while and :return.

closes: #15684

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

runtime/syntax/testdir/input/vim9_expr.vim

@@ -0,0 +1,20 @@
+vim9script
+# Vim9-script expressions
+
+
+# Command {expr} arguments
+
+if true
+  echo true
+elseif false
+  echo false
+endif
+
+while true
+  break
+endwhile
+
+def Foo(): bool
+  return true
+enddef
+