Problem:  Logic computing the new height of the modified area does not
          take into account virtual lines attached to a folded line.

Solution: Remove `hasFolding()` branch and let `plines_win_full()` do its job.

Problem:  An on_win-disabled decoration provider is left disabled for
          the on_buf callback during the next redraw (if the provider
          does not subscribe to on_end).

Solution: Move re-activation of the provider from after the on_end
          callback to before the on_start callback.

Problem:  inline word diff treats multibyte chars as word char
          (after 9.1.1243)
Solution: treat all non-alphanumeric characters as non-word characters
          (Yee Cheng Chin)

Previously inline word diff simply used Vim's definition of keyword to
determine what is a word, which leads to multi-byte character classes
such as emojis and CJK (Chinese/Japanese/Korean) characters all
classifying as word characters, leading to entire sentences being
grouped as a single word which does not provide meaningful information
in a diff highlight.

Fix this by treating all non-alphanumeric characters (with class number
above 2) as non-word characters, as there is usually no benefit in using
word diff on them. These include CJK characters, emojis, and also
subscript/superscript numbers. Meanwhile, multi-byte characters like
Cyrillic and Greek letters will still continue to considered as words.

Note that this is slightly inconsistent with how words are defined
elsewhere, as Vim usually considers any character with class >=2 to be
a "word".

related: vim/vim#16881 (diff inline highlight)
closes: vim/vim#17050

https://github.com/vim/vim/commit/9aa120f7ada592ed03b37f4de8ee413c5385f123

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

Problem:  Successive autocmd verbose messages may be emitted without a kind.

Solution: Always set the kind when preparing to emit a verbose message.

Problem:  Redrawing during a substitute confirm prompt causes the match
          highlight to disappear.
Solution: Unset `highlight_match` after the prompt has returned.
          Use global highlight definitions in searchhl_spec.lua.

Problem:  Negative window and grid height with 'winbar'.
Solution: Clamp the height when subtracting the 'winbar' height.

Problem:  Inline virtual text placed in a decor provider callback
          invalidates `w_virtcol`, which must be valid for `win_line()`.
Solution: Call `validate_virtcol()` after "line" decor provider callbacks.

Problem:  Scrolling up does not adjust `w_topline` for concealed lines
          directly above it, resulting in (non-visual) asymmetry when
          scrolling up/down.
Solution: Adjust `w_topline` for concealed lines after scrolling up.