Simple Flow Typing for Unit

[AugustNagro]

It is really cool that Scala 3 supports flow-sensitive typing for null.

It would also be helpful to have flow-typing for Unit. In Scala.js Unit maps to JavaScript undefined, and often when modeling npm library types you will have fields that may be undefined.

For example this works in Scala 3:

val x: Int | Null = ???

if x == null then println("was null")
else println(x + 1)

But not this:

val x: Int | Unit = ???

if x == () then println("was undefined")
else println(x + 1)

The current solution in Scala.js is to use js.UndefOr, but I think it may be nicer to use Scala 3 union types.

If this is something that could be accepted with a PR I could try my hand at implementing.

[smarter]

WDYT @sjrd?

[sjrd]

Oh I'm all for it, of course. I was already suggesting this when we started discussing flow typing for null, back when we were still physically talking around a table...

[AugustNagro]

I spent some time looking at this PR, creating a class Unitables.scala & started updating all relevant places.

But it seems like a bigger refactoring & codebase knowledge than I have the bandwidth for is required. Sorry guys.