feat: warn about wrong 'dir' values

[teto]

Warn about non-existing directories to help the user debug its config.

Feel free to edit/do anything to this PR

[github-actions]

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[mrcjkb]

Looks good to me.
At a later time, we could change the warning to Run ':checkhealth rocks-dev.nvim' and add health checks (like we do in rocks-config).

This will still try to load the plugin if the directory is not found. Do we want to skip that?

[teto]

This will still try to load the plugin if the directory is not found. Do we want to skip that?

I think it's fine. If user wants to restore the folder after neovim's start, it might work again without having to restart neovim.
I agree with the checkhealth, would be nice to have but this addresses the most urgent part.