Check for C# extension dependencies at runtime

[dbrattli]

Why

We should not have a hard dependency on ms-dotnettools.csharp since that makes it impossible to use Ionide with forks os VScode e.g Cursor. This PR does the check at runtime instead adding recommended extensions to the Extension Pack tab instead.

Fixes #2087

How

• Check for valid C# extensions at runtime instead of in the manifest
• Suggest valid dependencies ast Extension Pack instead to allow for easy navigation to required C# extension
• Give error message if no valid C# extension can be found

[baronfel]

This seems like an easy substitution - really the only thing we use the C# extension for today is debugging related, so another way this could be handled instead of blocking is to

• do capabilities-based detection (probing for the 'coreclr' debug type)
• in the debugging code, if debugging isn't available throw a message/notification to the user directing them to the extensions panel for the C# extension that's present in their environment

[baronfel]

Thanks for this - we'll get a release out later today US time