fix(lua): stop pending highlight.on_yank timer, if any (neovim#18824)

When yanking another range while previous yank is still highlighted, the
pending timer could clear the highlight almost immediately (especially
when using larger `timeout`, i.e. 2000)

runtime/lua/vim/highlight.lua

@@ -63,7 +63,8 @@ function M.range(bufnr, ns, higroup, start, finish, opts)
   end
 end
 
-local yank_ns = api.nvim_create_namespace("hlyank")
+local yank_ns = api.nvim_create_namespace('hlyank')
+local yank_timer
 --- Highlight the yanked region
 ---
 --- use from init.vim via
@@ -113,6 +114,9 @@ function M.on_yank(opts)
 
   local bufnr = api.nvim_get_current_buf()
   api.nvim_buf_clear_namespace(bufnr, yank_ns, 0, -1)
+  if yank_timer then
+    yank_timer:close()
+  end
 
   local pos1 = vim.fn.getpos("'[")
   local pos2 = vim.fn.getpos("']")
@@ -129,7 +133,8 @@ function M.on_yank(opts)
     { regtype = event.regtype, inclusive = event.inclusive, priority = M.priorities.user }
   )
 
-  vim.defer_fn(function()
+  yank_timer = vim.defer_fn(function()
+    yank_timer = nil
     if api.nvim_buf_is_valid(bufnr) then
       api.nvim_buf_clear_namespace(bufnr, yank_ns, 0, -1)
     end