Better diagnostics for nullness inference related errors

[psfinaki]

I think it would be good to be more explicit about nullness inference happening behind the scenes.

For the code

let xs = [| ""; ""; null |]

we'll get

Nullness warning: The type 'string' does not support 'null'

It would be helpful to have something like

"xs was inferred to have the type string[] but got null among its values. Either explicitly specify null in the return type ((string | null)[]), or remove null from the collection - or suppress the warning but no please don't."

Originally posted by @psfinaki in https://github.com/dotnet/fsharp/pull/15181#discussion_r1679527189

[vzarytovskii]

We don't differentiate between collections and other types. string and string array are no different for us. This would probably be better solved in analyzers, rather than typechecker.

[T-Gro]

Related to: https://github.com/dotnet/fsharp/issues/17409

[T-Gro]

Option: Specialize type unification for list/array/sequence builders.

[T-Gro]

Diagnostics can offer: Explicit annotation or "be first" rule.