Add flag to control nullable feature

[jcouv]

For the prototype, we're using LangVersion as the flag for both language version and nullable annotations.
At a minimum, we need an ability (separate from LangVersion) to disable nullable warnings.

From discussions with Chuck, we would have to decide whether this flag affects the symbols that we produce:

• pretty clearly, we would want to emit string (no nullable annotations)
• you type string? in source (with nullable warnings disabled), does QuickInfo show you string? back?
• maybe the IDE should grey out any ?, since it is unnecessary. We can have a code fixer to remove the ? or to turn on nullable warnings.

Some related questions, when the feature is turned off:

• What if you use [NonNullTypes] but the nullable feature is turned off? Warning?
• What about using ? or !?

Tagging @cston @AlekseyTs

Relates to #26199

[AlekseyTs]

I don't think LanguageVersion should affect "shape" of symbols. Otherwise, sharing of symbols is going to be affected in a negative way, as well as this is likely to complicate the Reference manager and make it dependent on ParserOptions, which never happened before.

[jcouv]

From LDM discussion today:

• we'll use a separate flag (not warning waves) to control the feature
• the flag implicitly changes top-level NonNullTypes context to be true
• the flag enables the flow analysis warnings
• if you use any nullability-related feature (!, ? or [NonNullTypes]) and the flag isn't set, then we produce a new warning ("please enable the feature")

[cston]

Updated LDM decision to use [NonNullTypes] to enable warnings.