[Feature] Convert Project to C# 9

[ravero]

This a feature suggestion based on this and [this]() blogs posts.

The ideia is to add a command on the project context menu to with the Convert Project to C# 9.0 to automatically configure the project in question to be compiled with C# 9.0.

The pre-conditions are:

• Projet types must be Xamarin.Android, Xamarin.iOS or .NET Standard Projects that have the Xamarin.Forms library.

The actions are:

• Add/Update the <langversion> element on the .csproj file to 9.0 explicitly
• If the project is a .NET Standard project add the IsExternalInit empty class declaration on the root of the project to suppress the erros of the absence of this class

[ravero]

Feature completed. Waiting on @matthewrdev review.

[matthewrdev]

@ravero Looks like you implemented this feature last year :)

Closing this issue.