feat(extmarks): add virt_text_repeat_linebreak flag (#26625)

Problem:  Unable to predict which byte-offset to place virtual text to
          make it repeat visually in the wrapped part of a line.
Solution: Add a flag to nvim_buf_set_extmark() that causes virtual
          text to repeat in wrapped lines.

runtime/doc/api.txt

@@ -2696,6 +2696,8 @@ nvim_buf_set_extmark({buffer}, {ns_id}, {line}, {col}, {*opts})
                     text is selected or hidden because of scrolling with
                     'nowrap' or 'smoothscroll'. Currently only affects
                     "overlay" virt_text.
+                  • virt_text_repeat_linebreak : repeat the virtual text on
+                    wrapped lines.
                   • hl_mode : control how highlights are combined with the
                     highlights of the text. Currently only affects virt_text
                     highlights, but might affect `hl_group` in later versions.

runtime/doc/news.txt

@@ -339,11 +339,10 @@ The following changes to existing APIs or features add new behavior.
   In addition, |nvim_buf_get_extmarks()| has gained an "overlap" option to
   return such ranges even if they started before the specified position.
 
-• Extmarks can opt-out of precise undo tracking using the new "undo_restore"
-  flag to |nvim_buf_set_extmark()|
-
-• Extmarks can be automatically hidden or removed using the new "invalidate"
-  flag to |nvim_buf_set_extmark()|
+• The following flags were added to |nvim_buf_set_extmark()|:
+  - "undo_restore": opt-out extmarks of precise undo tracking.
+  - "invalidate": automatically hide or delete extmarks.
+  - "virt_text_repeat_linebreak": repeat virtual text on wrapped lines.
 
 • LSP hover and signature help now use Treesitter for highlighting of Markdown
   content.

runtime/lua/vim/_meta/api.lua

@@ -519,6 +519,8 @@ function vim.api.nvim_buf_line_count(buffer) end
 ---                 text is selected or hidden because of scrolling with
 ---                 'nowrap' or 'smoothscroll'. Currently only affects
 ---                 "overlay" virt_text.
+---               • virt_text_repeat_linebreak : repeat the virtual text on
+---                 wrapped lines.
 ---               • hl_mode : control how highlights are combined with the
 ---                 highlights of the text. Currently only affects virt_text
 ---                 highlights, but might affect `hl_group` in later versions.

runtime/lua/vim/_meta/api_keysets.lua

@@ -251,6 +251,7 @@ error('Cannot require a meta file')
 --- @field virt_text_pos? string
 --- @field virt_text_win_col? integer
 --- @field virt_text_hide? boolean
+--- @field virt_text_repeat_linebreak? boolean
 --- @field hl_eol? boolean
 --- @field hl_mode? string
 --- @field invalidate? boolean