fix(api/win_get_config): include z-index

* Add buffer information to tabline_update

Most terminal implementations of the tabline display buffer and tab
information. Many neovim-qt users disable GuiTabline because it lacks
functionality provided in the terminal implementation.

The tabline_update event should include buffer information too, so client GUIs
can display rich useful tabs.

Add `borders = "rounded"` preset for `nvim_open_win`, equivalent to

  border = {"╭", "─", "╮", "│", "╯", "─", "╰", "│"}

Also add undocumented "solid" preset to docs.

Problem:    Spell suggestions insufficiently tested.
Solution:   Add spell suggestion tests. (Dominique Pelle, closes vim/vim#5398)
https://github.com/vim/vim/commit/e9a8d1f9adaf4599b5a7923f8db8e207ed6e7eca

Requires latest en.utf-8.spl from
https://ftp.nluug.nl/pub/vim/runtime/spell/.

Include the following patch because patch v8.2.0946 was merged:

vim-patch:8.2.0948: spell test fails

Problem:    Spell test fails.
Solution:   Adjust expected text of the prompt.
https://github.com/vim/vim/commit/d281b7c227bc4c78813fdc297ccee4b2cad7e605

Problem:    There is :spellwrong and :spellgood but not :spellrare.
Solution:   Add :spellrare. (Martin Tournoij, closes vim/vim#4291)
https://github.com/vim/vim/commit/08cc374dabd2a02785129fa1c0100f7745c244ad

Refs #12985

Problem:    Cannot use "q" to cancel a number prompt.
Solution:   Recognize "q" instead of ignoring it.
https://github.com/vim/vim/commit/eebd555733491cb55b9f30fe28772c0fd0ebacf7

[WIP] z-index!