Add template for dotnet.config to dotnet new

[Youssef1313]

Original issue by @nohwnd in microsoft/testfx#5207:

Summary

testing platform new ux depends on placing dotnet.config file with content:

[dotnet.test:runner]
name= "Microsoft.Testing.Platform"

Add template to dotnet new to create the file, with option to specify the runner. To ease > onboarding to the new UX.

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Comments

By @Youssef1313

with option to specify the runner.

Is the option required? Or will we have to choose a "default" when not provided?

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By @nohwnd

I am under the impression that dotnet.config will be a future thing that dotnet will use for more things, and so we should not make that option required. Even if we are the first one to adopt it.

Without the option it should make an empty file probably, which is useless now, but can be later populated by some defaults as the needs for dotnet.config increase in dotnet itself.

I am thinking about this the same way as using dotnet new globaljson and dotnet new globaljson --roll-forward where roll forward has some implicit default value that the default file does not mention, but there is still option to set it that makes authoring the file more convenient for the user. Especially when following a tutorial.

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By @Evangelink

@nohwnd It might be worth to move this issue to SDK repo as it helps SDK team to be aware of changes.

Pinging @baronfel for awareness. BTW Chet, what's the status of the discussion for the file format? Preview 2 is already out so we need to be certain of what we can use before it's getting too late for us to be allowed to change.