[API Compat] add rule to make sure that a member is not made more visible

[safern]

This rule should make sure that a member that is protected or protected internal in the implementation is the same in the contract and is not promoted to public.

[smasher164]

https://docs.microsoft.com/en-us/dotnet/core/compatibility/ mentions that
✔️ ALLOWED: Expanding the visibility of a type
✔️ ALLOWED: Restricting the visibility of a protected member when the type has no accessible (public or protected) constructors, or the type is sealed.
❌ DISALLOWED: Reducing the visibility of a type

so I think the rule should be mostly checking for the opposite of what is in the first comment.

[KalleOlaviNiemitalo]

Those compatibility rules are for "Modifications to the public contract", but the description of this issue is in terms of "contract" vs. "implementation". If the contract assembly defines a member as public but the implementation assembly defines it as protected, then a non-derived class in a project that references the contract can access the member at compile time, but the access will fail at run time when the implementation is used. So the description is correct although written as if the implementation comes before the contract.