Allow applying `[<AutoSerializable(false)>]` on assemblies

# Allow applying `[]` on assemblies

I propose we allow the `FSharp.Core.AutoSerializableAttribute` to be applied in assemblies, which will make the compiler entirely skip automatically marking all types as `[]`.

The existing way of approaching this problem in F# is manually applying the attribute on each and every type, and still it might not catch some compiler-generated ones.

## Pros and Cons

The advantages of making this adjustment to F# are easier discouraging and breaking off from the insecure `BinaryFormatter`. Moreover library authors will be able to prevent unforeseen backwards compatibility issues by easily marking all their types as non-serializable.

The disadvantages of making this adjustment to F# are none; this change would not impact existing codebases. Furthermore `BinaryFormatter` is not used in `FSharp.Core` at all so it won't break anything there.

## Extra information

Estimated cost (XS, S, M, L, XL, XXL): S

Related suggestions: _N/A_

Developers could mark _only_ specific types as serializable by applying `[]` on these types and `[]` on the assembly.

## Affidavit (please submit!)

Please tick this by placing a cross in the box:
* [X] This is not a question (e.g. like one you might ask on [stackoverflow](http://stackoverflow.com)) and I have searched stackoverflow for discussions of this issue
* [X] I have [searched both open and closed suggestions on this site](http://github.com/fsharp/fslang-suggestions/issues) and believe this is not a duplicate
* [X] This is not something which has obviously "already been decided" in previous versions of F#. If you're questioning a fundamental design decision that has obviously already been taken (e.g. "Make F# untyped") then please don't submit it.

Please tick all that apply:
* [X] This is not a breaking change to the F# language design
* [ ] I or my company would be willing to help implement and/or test this

## For Readers

If you would like to see this issue implemented, please click the :+1: emoji on this issue. These counts are used to generally order the suggestions by engagement.

Start by locating the existing handling of FSharp.Core.AutoSerializableAttribute and the compiler logic that automatically marks types with Serializable. Check how assembly-level attributes are represented and how compiler-generated types are covered. Done means an assembly-level AutoSerializable(false) prevents automatic serialization marking while explicit Serializable attributes still work, with corresponding tests.

Written by the indexing model from the issue text.