runtime(vim): Update base-syntax, match types in Vim9 variable declarations

Match types in Vim9 variable declarations.

Match Vim9 boolean and null literals. These are not matched in all
contexts yet.

related: #15277

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/testdir/input/vim9_keymap.vim b/runtime/syntax/testdir/input/vim9_keymap.vim
index a69b723..1c59e56 100644
--- a/runtime/syntax/testdir/input/vim9_keymap.vim
+++ b/runtime/syntax/testdir/input/vim9_keymap.vim
@@ -4,7 +4,7 @@
 
 scriptencoding utf-8
 
-let b:keymap_name = "syntax-test"
+b:keymap_name = "syntax-test"
 
 loadkeymap