runtime(go): use :term for keywordprg for nvim/gvim

runtime(go): use :term for keywordprg for nvim/gvim

Problem:
- The document from `go doc` can be very long, and you can scroll if
  using `!` to run shell command in Gvim.
- I realize that I didn't fully mimic behavior of default keywordprg
  in Nvim in the last commit.

Solution:
- Use builtin terminal for keywordprg in Gvim
- In Nvim (both TUI and GUI), it should mimic the behavior of Vim
  `:term`, `:Man`, and `:help`

closes: #16911

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Phạm Bình An <phambinhanctb2004@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>