fix: avoid custom autocmd conflicts with the help popup buffer

Issue
=====

If the user has set up custom autocmds that trigger for `help` buffers,
the yazi.nvim help popup buffer will be affected by them. I noticed this
after setting the following autocmd:

```lua
vim.api.nvim_create_autocmd("FileType", {
  -- always open help buffers in a vertical split
  pattern = { "help", "man" },
  command = "wincmd L",
})
```

Solution
========

Only use the `syntax` option to set the filetype of the help buffer to
`help`. This way, the user's custom autocmds will not affect the help
popup buffer.

lua/yazi/config.lua

@@ -231,7 +231,7 @@ function M.set_keymappings(yazi_buffer, config, context)
         "version *" .. require("yazi").version .. "*",
       })
 
-      vim.api.nvim_set_option_value("filetype", "help", { buf = help_buffer })
+      vim.api.nvim_set_option_value("syntax", "help", { buf = help_buffer })
       vim.api.nvim_set_option_value("modifiable", false, { buf = help_buffer })
 
       local function close_help()