runtime(vim): Update base-syntax, match shebang lines (#15011)

Match shebang lines in Vim9 and legacy script.

Mark these as an error if they appear anywhere other than the first line
of a legacy-script file.  In Vim9 script these match as normal line
comments rather than an error.

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

runtime/syntax/testdir/input/vim_shebang.vim

@@ -0,0 +1,5 @@
+#!/usr/bin/env vim -S
+
+" Vim shebang line
+
+#!/usr/bin/env vim -S