Before calling "attach" a screen object is just a dummy container for
(row, col) values whose purpose is to be sent as part of the "attach"
function call anyway.

Just create the screen in an attached state directly. Keep the complete
(row, col, options) config together. It is still completely valid to
later detach and re-attach as needed, including to another session.

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

Work on https://github.com/neovim/neovim/issues/27004.

Problem: "NOTE"s, inline Vim script code, and links ending in digits may not be
highlighted correctly within the :Tutor.

Solution: set an explicit value for ":syntax iskeyword" that includes digits. Do
it after ":syntax include"s, so the included syntax/sh.vim doesn't mess with the
value.

Increase screen test width so all text within the conclusion section is visible.

Co-authored-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>

Problem:
Some steps in :Tutor don't work on Windows.

Solution:
Add support for `{unix:...,win:...}` format and transform the Tutor contents
depending on the platform.
Fix https://github.com/neovim/neovim/issues/24166