Shorter syntax for optional return types

[zakjan]

Currently there is shorter syntax for optional named fields, the question mark next to the field name, field?: string means field: string | undefined.

Could this syntax be extended from named fields to all types in general? For example string? would mean string | undefined.

It would allow using this shorter syntax in function return types:

Proposed syntax:

function identity(value: string?): string? {
    return value;
}

Current syntax:

function identity(value?: string): string | undefined {
    return value;
}

(This should stay supported for backward compatibility.)

[aluanhaddad]

@zakjan have fun in #7426