Add support for extending partial methods

[RexJaeschke]

No description provided.

[KalleOlaviNiemitalo]

partial seems to be also recognized when it immediately precedes async:

using System.Threading.Tasks;
public partial class C {
    private partial Task M();
    private partial async Task M() {}
}

but I don't know which version of C# this requires.

[KalleOlaviNiemitalo]

Please revert this change; public async int M() { throw null; } should still be disallowed, even though it is not a restricted partial method (nor an unrestricted partial method).

[KalleOlaviNiemitalo]

Is this error specified?

public static partial class C {
    // error CS8796: Partial method 'C.M(out int)' must have accessibility modifiers because it has a non-void return type.
    partial int M(out int p);
    partial int M(out int p) { return p = 0; }
}

[KalleOlaviNiemitalo]

public partial class C {
    static partial void M(int i = 1);
    static partial void M(int j = 2) {}
    
    static void N() {
        // OK, the name of the parameter is "i", not "j".
        M(i: 4);
        
        // Calls M(1).  The default value 2 in the implementing declaration
        // has no effect on this call.
        M();
    }
}

[KalleOlaviNiemitalo]

private extern partial void M(); should also be an implementing partial method declaration even though method_body is a semicolon.

[KalleOlaviNiemitalo]

The async and extern modifiers should disallowed on the defining partial method declaration, but allowed on the implementing partial method declaration, thus not required to match.