Turn Nullable on for new projects

[terrajobst]

We want to set <Nullable>Enabled</Nullable> for new projects.

We probably don't want to infer this as part of the TFM and instead make it part of the project template. Otherwise retargeting becomes a major hurdle.

[samsp-msft]

@cartermp - who is the dev for this?

[cartermp]

Nobody yet, but it's trivial to do the work to change the templates. The issue is more if everything else - libraries, app frameworks, etc. - can handle it being on by default for new projects.

[terrajobst]

@KathleenDollard AFAIK this is completed now, right?