Allow Type Augmentation Extension on composite types

I propose we allow adding intrinsic/optional extensions on type augmentation or composite data types. This makes it a bit more natural to extend composite types analogous to using intrinsic/optional extension on single data type.

Will stress that this is coming purely to make writing type extensions more natural and doesn't care much about interoperability with C#. Type aliases would also simply expand to the underlying type and create extension methods.

```fsharp
type int list with
member this.Print() =
printfn "%A" this
```

The existing way of approaching this problem in F# is using Extension methods:

```fsharp
[]
type IntExtensions =
[]
static member Print(_this: int list) =
printfn "%A" _this
```

## Pros and Cons

The advantages of making this adjustment to F# are able
* Consistency with existing F# way to declare intrinsic or optional type extensions.
* Trait like syntax.

The disadvantages of making this adjustment to F# are
* Multiple ways to achieve the same thing
* Brings question on if all extensions essentially compile to C# extension methods?

## Extra information

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

Related suggestions: (put links to related suggestions here)

## 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.

The payload names no implementation files or tests; start by reading the proposed composite-type syntax and the existing intrinsic/optional-extension and Extension-method examples. Done would mean an agreed F# language design and an implementation plan for the requested behavior.

Written by the indexing model from the issue text.