[Nullable Reference Types] Compiler directives for nullable reference types

[BillWagner]

Specify the compiler directive #nullable that controls the interpretation of reference types and nullability. The effect of the directives is as follows:

• #nullable disable: Sets the nullable annotation and warning contexts to disabled.
• #nullable enable: Sets the nullable annotation and warning contexts to enabled.
• #nullable restore: Restores the nullable annotation and warning contexts to project settings.
• #nullable disable annotations: Sets the nullable annotation context to disabled.
• #nullable enable annotations: Sets the nullable annotation context to enabled.
• #nullable restore annotations: Restores the nullable annotation context to project settings.
• #nullable disable warnings: Sets the nullable warning context to disabled.
• #nullable enable warnings: Sets the nullable warning context to enabled.
• #nullable restore warnings: Restores the nullable warning context to project settings.

In addition, the spec should define the behavior of reference types in those contexts.

---

Associated WorkItem - 248771