feat: multibuffer preview support for inccommand

Problem:    Insufficient testing for reading/writing files.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#6257)
            Add "ui_delay" to test_override() and use it for the CTRL-O test.
https://github.com/vim/vim/commit/b340baed9f7fc1c19a0742e2214d54982190c15e

Omit test_override().
Reorder test_writefile.vim to match Vim.

Allows preview callbacks to modify multiple buffers in order to show the
preview. Previously, if multiple buffers were modified, only the current
buffer would have its state restored. After this change, all buffers
have their state restored after preview.

Closes #19103.

This reverts commit c4c74c3883aa3122c0c877ca8dd7b26beb5cc4aa.

LibUV already gives an error for this, so this isn't needed.

fix(api): make nvim_set_hl(ns=0, ...) redraw screen properly

fixes #18160

fixes #19823

These were just added to avoid churn when changing the default
of 'display'. To simplify message handling logic, we might want
to remove support for printing messages in default_grid later on.
This would allow things like printing error messages safely in the
middle of redraw, or a future graduation of the 'multigrid' feature.

- reimplement 'winhl' in terms of highlight namespaces
- check for EOF in screen tests (to indicate a likely crash)