runtime(vim): Update syntax file and syntax test (#13632)

Add missing assignment operators (:let*=, :let/= and :let%=).

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

runtime/syntax/testdir/input/vim_variables.vim

@@ -0,0 +1,11 @@
+" Vim variable highlighting
+
+let foo = expr
+
+let foo += expr
+let foo -= expr
+let foo *= expr
+let foo /= expr
+let foo %= expr
+let foo .= expr
+let foo ..= expr