[WIP] C# 14: Support extension blocks.

[michaelnebel]

In this PR we introduce support for extension types (blocks). The feature is explained in detail here.
The feature generalises extensions to operators and properties.

Here is a small example of an declaration and how it can be invoked.

public static class MyExtensions
{
    extension(string s)
    {
        public bool IsValid() { return s is not null; }
    }
}

public class A
{
    public void M()
    {
        var s = "Hello World";
        s.IsValid(); // Call to extension method.
        MyExtensions.IsValid(s) // Call to compiler generated static method.
    }
}

It turns out that Roslyn generates a static method corresponding to the extension method. To avoid extracting multiple methods with identical bodies (which would further complicate the QL implementation) we

• Only extract the IsValid() method, which has the qualified name MyExtensions.extension(string).IsValid.
• Add a parameter s to IsValid() corresponding to the receiver parameter s. This needs to be synthesised as we can't re-use the parameter from the extension declaration.
• Replace invocations of MyExtensions.IsValid with MyExtensions.extensions(string).IsValid.