Change msbuild property to "Nullable"

[jcouv]

From LDM today (5/1/2019), we landed on Nullable rather than NullableContextOptions or NullableReferenceTypes.
The property should be a string, since it will accept "enable", "disable" and a couple other values.

Tagging @KathleenDollard @rainersigwald to confirm the name is okay in the msbuild ecosystem.

[jcouv]

@rainersigwald Do you see any issue with naming this property "Nullable"?
Previously, we picked longer names thinking we should avoid conflicts, but Jared found no github project referencing a property named "Nullable". LDM preferred "Nullable" because it's easier to remember.

[rainersigwald]

No objection here. It's impossible to guarantee that someone somewhere won't already be using a property named Nullable, but I didn't turn up anything widespread on GitHub either, so the risk seems acceptable.

The only other thing I'd mention is that other project types use MSBuild, including C++. Having names that refer to broad concepts that apply in multiple languages/compilers/tools increases the possibility of user confusion when it doesn't work in $non_csharp_project_type_x. But that's not blocking. I think Nullable is fine.