Bring [<Erased>] attribute to FSharp.Core and add simple features around it

This spans from the discussion of https://github.com/fsharp/fslang-suggestions/issues/538 "Erased type-tagged anonymous union types", which is from 2017 and not seeming to be implemented.

I actually like the simpler idea by @voronoipotato in his comment, because that sounds very implentable. And that's not a language feature, but just a minor addition to FSharp.Core and compiler.

1. Why not to add `[]` to F# core?
- Definition: It's an instruction to the compiler that the type can be simplified by compile-time reduction.
- What it actually does in practice depends on the compiler target (IL/JS/...)
- Fable can replace its attribute with FSharp.Core version.

2. As the first traditional F# compiler, the feature would be: Remove tagged single-case DUs:
- `[] type Phone = | Phone of string` could compile into `string`
- Benefit: Provide simpler C# interopability usage.
- Benefit: Provide more performant runtime execution while still maintaining the type safety for the developer.
- A compile-time thing, a little bit like ReflectedDefinitionAttribute. If a user breaks their own code with it (e.g. calling the type via another F# dll via ProjectReference instead of NuGet/dll-reference), it's their own problem. (Could it be erased in whole sln or is that too complex?)