fix(treesitter): don't open fold when o/O adds a line below #28709

Problem:
`o`-ing on a folded line opens the fold, because the new line gets the
fold level from the above line (level '='), which extends the fold to
the new line. `O` has a similar problem when run on the line below a
fold.

Solution:
Use -1 for the added line to get the lower level from the above/below
line.

runtime/lua/vim/treesitter/_fold.lua

@@ -87,7 +87,7 @@ end
 ---@param srow integer
 ---@param erow integer 0-indexed, exclusive
 function FoldInfo:add_range(srow, erow)
-  list_insert(self.levels, srow + 1, erow, '=')
+  list_insert(self.levels, srow + 1, erow, -1)
   list_insert(self.levels0, srow + 1, erow, -1)
 end