runtime(vim): Improve heredoc handling for all embedded scripts

runtime(vim): Improve heredoc handling for all embedded scripts

* Improve heredoc handling
  - Support "trim" for all the embedded scripts.
  - Check the indent of "trim" for "let" and all the embedded scripts.
* Update missing part of vim.vim.base in the commit
  d164f2a521f8e52e587727657fb1c19e9a25f32a.
* Update gen_syntax_vim.vim to catch up with 9.1.0685's source code.

closes: #15542

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