GDScript: Disallow type inference with untyped initializer

[vonagam]

func test():
  var untyped = 1
  var inferred := untyped # complains now:
  # Cannot infer the type of "inferred" variable because the value doesn't have a set type.

inferred will be confident in its type, while in reality untyped can end up being a different thing. (Not in such simple example, but if untyped is a member variable or an argument.)

Related to discussions in #69163 and #70733.

[dalexeev]

I tested this PR, for variables and parameters it works correctly. We probably need to add tests for the parameters as well.

Notes:

1. It's not directly related to this PR, but I'm surprised that := works for constants at all. I thought that this is forbidden at the parser level.

const A = 1
const B := A

2. The error does not occur if, for example, the initializer is a method that returns Variant:

var a = 1
var b1 := a # Error
var b2 := self.a # Error
var b3 := get("a") # OK (Variant)

This can be confusing for users.

[vonagam]

Logic for constants was that they always do infer with or without a colon. Did not want to change much of behaviors, so I kept this logic during unification.

As for variables, we discussed this at last meeting, currently there is a distinction between typed Variant and untyped Variant. You cannot infer on untyped Variant (as message correctly says there is no set type), but you can on Variant (since the type is set). Before unification it wasn't like that.

I propose to add warning with default level of error for this, so that by default your b3 will complain, but user can disable those.

[akien-mga]

Thanks!