feat(luahl): add priority mechanism

Base priority is 0x1000, in order to stay kinda backward compatible.
Also set tree-sitter default highlight to 100 (middle-ish value)

runtime/lua/vim/treesitter/highlighter.lua

@@ -219,7 +219,8 @@ local function on_line_impl(self, buf, line)
         a.nvim_buf_set_extmark(buf, ns, start_row, start_col,
                                { end_line = end_row, end_col = end_col,
                                  hl_group = hl,
-                                 ephemeral = true
+                                 ephemeral = true,
+                                 priority = 100 -- Low but leaves room below
                                 })
       end
       if start_row > line then