feat!: use async for cargo root dir

[saghen]

Using snacks.profile and profile.nvim, I noticed a 25ms delay when opening rust files, primarily due to the :wait() on vim.system call to get the cargo root directory. This PR changes the get_cargo_metadata call to use async instead for both the vim.system and vim.uv.fs_stat calls. Unfortunately, there's a breaking change to the config.root_dir option as a result, since the default implementation is now async.

• Neotest .root() needs to be updated
• No longer attaches to the first opened buffer

[github-actions]

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• Tested
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[saghen]

Brilliant, thanks @mrcjkb! Closed by 26ba138

[mrcjkb]

Thanks for your work @saghen 😄