Tweak Nameof slightly, add GetNameof() support to more expressions

[distributivgesetz]

Throwing is bad. Unwinding isn't healthy for tracking code errors. I refactored GetNameof() to return an optional string, which gets checked in the actual Nameof expression. There we can emit the error.

The arguments for GetNameof felt unnecessary since dmObject was unused, proc was used only once in Local, so I cut the arguments from the method. Let me know if I should keep it.

nameof() is also usable in constant contexts (eg. switch cases), so I added an override for TryAsConstant().

Also a few l-values didn't have any nameof support, so I added it in this PR.

[Hinaichigo]

When attempting to test this I get the following error:
C:...OpenDream\DMCompiler\DM\Expressions\Dereference.cs(295,45): error CS0246: The type or namespace name 'NotNullWhen' could not be found (are you missing a using directive or an assembly reference?)
I usually build OD using the dotnet build command, so maybe it's something that is normally included when you use an IDE like visual studio? Is there a workaround or some way to make it just werk with some kind of project setting?

[distributivgesetz]

I'm going to move the eval of a name from the expression emission to the building phase, and if nothing depends on the existence of Nameof then I'm down to removing the class to return a constant string. Nameof is a pretty bare class anyways and I don't feel like its existence justifies having to include context parameters into TryAsConstant (which is why I removed the parameters from GetNameof)

[distributivgesetz]

When attempting to test this I get the following error: C:...OpenDream\DMCompiler\DM\Expressions\Dereference.cs(295,45): error CS0246: The type or namespace name 'NotNullWhen' could not be found (are you missing a using directive or an assembly reference?)

Could be a problem with your RT submodule because the annotations package is referenced there. no idea otherwise.