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.

src/nvim/api/extmark.c

@@ -391,6 +391,8 @@ Array nvim_buf_get_extmarks(Buffer buffer, Integer ns_id, Object start, Object e
 ///                                  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`
@@ -613,7 +615,8 @@ Integer nvim_buf_set_extmark(Buffer buffer, Integer ns_id, Integer line, Integer
   }
 
   hl.flags |= opts->hl_eol ? kSHHlEol : 0;
-  virt_text.flags |= opts->virt_text_hide ? kVTHide : 0;
+  virt_text.flags |= ((opts->virt_text_hide ? kVTHide : 0)
+                      | (opts->virt_text_repeat_linebreak ? kVTRepeatLinebreak : 0));
 
   if (HAS_KEY(opts, set_extmark, hl_mode)) {
     String str = opts->hl_mode;

src/nvim/api/keysets_defs.h

@@ -33,6 +33,7 @@ typedef struct {
   String virt_text_pos;
   Integer virt_text_win_col;
   Boolean virt_text_hide;
+  Boolean virt_text_repeat_linebreak;
   Boolean hl_eol;
   String hl_mode;
   Boolean invalidate;