feat(diff): merge adjacent blocks using inline:word (#37085)

vim-patch:9.2.0174: diff: inline word-diffs can be fragmented

Problem:  When using 'diffopt=inline:word', lines were excessively
          fragmented with punctuation creating separate highlight
          blocks, making it harder to read the diffs.
Solution: Added 'diff_refine_inline_word_highlight()' to merge
          adjacent diff blocks that are separated by small gaps of
          non-word characters (up to 5 bytes by default) (HarshK97).

When using inline:word diff mode, adjacent changed words separated by
punctuation or whitespace are now merged into a single highlight block
if the gap between them contains fewer than 5 non-word characters.

This creates more readable diffs and closely matches GitHub's own diff
display.

closes: vim/vim#19098

42c6686c78

src/nvim/options.lua

@@ -2459,7 +2459,10 @@ local options = {
         				difference.  Non-alphanumeric
         				multi-byte characters such as emoji
         				and CJK characters are considered
-        				individual words.
+        				individual words.  Small gaps of
+        				non-word characters (5 bytes or less)
+        				between changes are merged into a
+        				single highlight block.
 
         	internal	Use the internal diff library.  This is
         			ignored when 'diffexpr' is set.  *E960*