Make it optional to repeat the constraints in extensions

**I propose we** make it optional to repeat the constraints on type parameters when writing an extension on a generic type.

```fsharp
type Foo<'T when 'T : struct> = { x: 'T }

// OK
type Foo<'T when 'T : struct> with
member this.Y = 1

// Currently: error FS0957: One or more of the declared type parameters for this type extension
// have a missing or wrong type constraint not matching the original type constraints on 'Foo<_>'
// Proposal: make this valid
type Foo<'T> with
member this.Z = 1
```

**The existing way of approaching this problem in F# is** to repeat all constraints. However this is not always possible:

This is actually causing a regression in F# 9, as mentioned [here](https://hachyderm.io/@BentRs/113476256823511243). The problem is that .NET 9 introduces a new constraint `allows ref struct` which cannot (yet) be expressed in F#. So types that have this constraint currently cannot be extended. This is especially problematic because this constraint has been added to existing standard library types such as `IEnumerable`.

```fsharp
// F# 8: OK
// F# 9: error FS0957
type System.Collections.Generic.IEnumerable<'T> with
member _.Foo = ()
```

## Pros and Cons

**The advantages of making this adjustment to F# are**
* more conciseness when extending a type that has constraints.
* working around the `allows ref struct` regression mentioned above, and avoiding similar future regressions.

**The disadvantages of making this adjustment to F# are**
* constraints are less explicit I guess?

## Extra information

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

**Related suggestions:** N/A

## Affidavit (please submit!)

Please tick these items 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] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to [the compiler and tooling repository](https://github.com/dotnet/fsharp)
* [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
* [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

Please tick all that apply:
* [x] This is not a breaking change to the F# language design
* [x] 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 reviewing the generic type-extension examples and the F# 9 regression involving `allows ref struct`, then examine the linked compiler and tooling repository for the language-design and implementation entry points. Done means extensions can omit repeated constraints while preserving existing constrained extensions and the shown `IEnumerable<'T>` case.

Written by the indexing model from the issue text.