extmarks/bufhl: reimplement using new marktree data structure

Add new "splice" interface for tracking buffer changes at the byte
level. This will later be reused for byte-resolution buffer updates.
(Implementation has been started, but using undocumented "_on_bytes"
option now as interface hasn't been finalized).

Use this interface to improve many edge cases of extmark adjustment.
Changed tests indicate previously incorrect behavior. Adding tests for
more edge cases will be follow-up work (overlaps on_bytes tests)

Don't consider creation/deletion of marks an undoable event by itself.
This behavior was never documented, and imposes  complexity for little gain.

Add nvim__buf_add_decoration temporary API for direct access to the new
implementation. This should be refactored into a proper API for
decorations, probably involving a huge dict.

fixes #11598

src/nvim/edit.c

@@ -1826,11 +1826,14 @@ change_indent (
 
     /* We only put back the new line up to the cursor */
     new_line[curwin->w_cursor.col] = NUL;
+    int new_col = curwin->w_cursor.col;
 
     // Put back original line
     ml_replace(curwin->w_cursor.lnum, orig_line, false);
     curwin->w_cursor.col = orig_col;
 
+    curbuf_splice_pending++;
+
     /* Backspace from cursor to start of line */
     backspace_until_column(0);
 
@@ -1838,13 +1841,16 @@ change_indent (
     ins_bytes(new_line);
 
     xfree(new_line);
-  }
 
-  // change_indent seems to bec called twice, this combination only triggers
-  // once for both calls
-  if (new_cursor_col - vcol != 0) {
-    extmark_col_adjust(curbuf, curwin->w_cursor.lnum, 0, 0, amount,
-                       kExtmarkUndo);
+    curbuf_splice_pending--;
+
+    // TODO(bfredl): test for crazy edge cases, like we stand on a TAB or
+    // something? does this even do the right text change then?
+    int delta = orig_col - new_col;
+    extmark_splice(curbuf, curwin->w_cursor.lnum-1, new_col,
+                   0, delta < 0 ? -delta : 0,
+                   0, delta > 0 ? delta : 0,
+                   kExtmarkUndo);
   }
 }