runtime(vim): Fix heredoc triggering misidentifcation of Vim9 script

runtime(vim): Fix heredoc triggering misidentifcation of Vim9 script

The following let-heredoc can cause legacy scripts to be misidentified
as Vim9 script if it appears early in the file.  Only match :vim9script
at the start of a line where it sensibly belongs.

let x =<< trim LINES
  vim9script
  ...
LINES

fixes:  #20647 (reported by Maxim Kim).
closes: #20654

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