BlazorWasm support

[TwoUnderscorez]

When trying to use IronPython3 from a Blazor web-assembly application, Microsoft.Scripting.Runtime.SharedIO throws an exception when trying to setup stdin, which is understandable, because there is none.

All I did is catch the exception and set the stream to Stream.Null.

[dnfadmin]

All CLA requirements met.

[TwoUnderscorez]

@slozier Is that better?

[TwoUnderscorez]

@slozier Sorry, I don't quite understand how this is a breaking change. If you would point me in the right direction I might be able to make it a non-breaking change.

[slozier]

@TwoUnderscorez I meant having ConsoleInputStream.Instance go from a field to a property is technically a binary breaking change (for example if assembly A uses a field from assembly B and you replace B with B' which contains a property instead, then A is broken). I don't think I'm really worried about breaking anyone with this particular scenario since the only consumer is in the same assembly - but it is a public type so it's not impossible...

An alternative approach would be to leave ConsoleInputStream.Instance as a field and change the constructor to not throw:

private ConsoleInputStream() {
    try {
        _input = Console.OpenStandardInput();
    } catch (PlatformNotSupportedException) {
        _input = Stream.Null;
    }
}

[TwoUnderscorez]

@slozier I could do that, but I would have to leave the catch block in ShareIO.InitializeInput because

_inputEncoding = Console.InputEncoding;
_inputReader = Console.In;

also throw PlatformNotSupportedException (maybe only one of them), which I personally think is kind of ugly. Would you like me to do what you've suggested or merge as is currently?

[slozier]

@TwoUnderscorez I think the PNSE try/catch you have in InitializeInput is fine, but I'd change the ConsoleInputStream to what I suggested instead.

[slozier]

Thanks for the PR!