fix(lua): highlight.on_yank can close timer in twice #18976

Steps to reproduce:

1. setting `vim.highlight.on_yank`
   ```
   vim.api.nvim_create_autocmd({ "TextYankPost" }, {
     pattern = { "*" },
     callback = function()
       vim.highlight.on_yank({ timeout = 200 })
     end,
   })
   ```
2. repeat typing `yeye` ...
3. causes the following error.
   ```
   Error executing vim.schedule lua callback: vim/_editor.lua:0: handle 0x01e96970 is already closing
   stack traceback:
           [C]: in function 'close'
           vim/_editor.lua: in function ''
           vim/_editor.lua: in function <vim/_editor.lua:0>
   ```

📝 Test result before fix:

    [----------] Global test environment setup.
    [----------] Running tests from test/functional/lua/highlight_spec.lua
    [ RUN      ] vim.highlight.on_yank does not show errors even if buffer is wiped before timeout: 15.07 ms OK
    [ RUN      ] vim.highlight.on_yank does not show errors even if executed between timeout and clearing highlight: 15.07 ms ERR
    test/helpers.lua:73: Expected objects to be the same.
    Passed in:
    (string) 'Error executing vim.schedule lua callback: vim/_editor.lua:0: handle 0x02025260 is already closing
    stack traceback:
            [C]: in function 'close'
            vim/_editor.lua: in function ''
            vim/_editor.lua: in function <vim/_editor.lua:0>'
    Expected:
    (string) ''

0 files changed, 0 insertions, 0 deletions