csharp: Solve problem of omnisharp server failing to download

[windoflake]

Fix issue #9181: #9181.

1. The root cause for the issue is that zed downloads the zip files and it used external command utility 'unzip' to extract to zip file. However, not all systems are equipped with it at default. So zed reports error like 'failed to start language server "OmniSharp": No such file or directory (os error 2)'. Once it fails, zed keep constantly downloading the zip file.
2. The solution is to use async_zip crate to extract the downloaded language zip file instead of calling 'unzip' command. So for any systems without unzip, zed should be working fine with it.

Release Notes:

• • Fixed #9181.

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[maxdeviant]

This looks like it’s touching non-zip code paths.

The tar and tar.gz paths should be left as-is.

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[windoflake]

This looks like it’s touching non-zip code paths.

The tar and tar.gz paths should be left as-is.

@maxdeviant , That's right. I've updated the code accordingly.
And there is no need to check whether the zip file is downloaded or not. The check logic should have been done in CsharpExtension::language_server_binary_path from csharp.cs. So I just change the unzip code.

[windoflake]

Sorry, I seem to overwrite my last push by a forcing push.
This is actually the commit that I changed according to the review comments. @maxdeviant