Allow extensions to provide custom variable/field/etc. name suggestions

[TahirAhmadov]

Currently, when you declare an variable of type List<Person>, it offers the name persons instead of just list, which makes sense.
However, there are other types, particularly those not in the BCL, where this kind of suggestion would be better than the default. For example, imagine a type like this:

struct EntityRef<T> { public int Id {get; set; } }

And when declaring a variable of type EntityRef<Person>, the suggestion should be personRef. I'm sure there are other types that people have in their code where this kind of enhancement would make sense.
As discussed on Discord, the easiest avenue would be to allow an extension to tap into a new extensibility point - given a ITypeSymbol, give me a list of suggestions, if non returned, continue with normal logic. (There would probably need to be a basic sanity check on the returned suggestions, like max count, max length of identifiers, allowed characters, etc.)

[CyrusNajmabadi]

Moved to #78996.