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

Also reformat tutor.tutor.json to use 2-space indent while at it.

* fix(tutor): adjust over-80ch lines and corresponding expect file
* fix(tutor): standardise indentation and formatting, add nowrap modeline

- unifies the formatting/layout, which was a bit inconsistent,
- adds a nowrap modeline

Since the tutor uses a lot of conceals, which are included in the character
count when calculating line wrapping, lines were breaking at what looked like
odd spots, which gives a poor first impression and lowered readability.

I have adjusted some lines to be over 80ch in the source, but once they're
rendered out with conceals, they're actually under 80, so even with nowrap we
don't visually extend past 80.

fix #15088

* Display ✓ or ✗ based on the line user sees
* Add vim-tutor-mode expectations to the lines marked with an arrow.
* Fix some existing expectations to behave predictably.

- tutor: emphasize K

closes #8426