Read default language for dotnet new from env var

[enricosada]

Read DOTNET_NEW_PREFERRED_LANG as default language for dotnet new.
If not set, the default continue to be C#

Currently to create F# projects, we need to specify -lang f# foreach dotnet new invocation.
It's really annoying, and atm dotnet new doesnt yet support set the preference property.

This option can be removed later, when dotnet new support set the default language (as normal config)

With this pr

set DOTNET_NEW_PREFERRED_LANG=F#
dotnet new console
dotnet new lib

/cc @cartermp

ref dotnet/sdk#4224

[nguerrera]

The bar for 3.1.200 is already past new features. 3.1.300 would be the soonest. We have an idea to have a configuration with aliasing like git config so then you could alias dotnet new to dotnet new -lang F#. Ideally, we wouldn't keep accumulating environment variables pending that feature. cc @KathleenDollard

[nguerrera]

That said, it may be too long to wait for the feature. I could be convinced that the scenario is so basic for non-C# developers that we need this as the interim solution.

[nguerrera]

I approve of the code change, but this cannot be merged until a 3.1.3xx branch opens up and this is retargeted to that.

[ghost]

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