[Implemented] Add Null to the language

In VB, we have only nothing, what translates to Default(of T).
Sometimes, this is a pain in the *** and we need a read NULL.

An example of usage:

Dim x = if(MyBool, True, Nothing)

Returns a Boolean, because nothing will translate to False
With the introduction of Null, we can write:

Dim x = if(MyBool, True, Null)

And this will return a nullable Boolean.

Implemented as of this week’s builds (2501 and up); was tracked as bugs://84131