Add `Deconstruct` to Discriminated Unions codegen for smoother C# consumption

I propose we add a C# convention Deconstruct method to Discriminated Unions. So that if I make a DU in F#

```fsharp
type Suit = Heart | Diamond | Club | Spade
type Card = {Rank:int; Suit:Suit}
type Hand = Card seq
type ScorableHand = EntireHand of Hand
| SplitHand of Hand * Hand
| TwoHand of Hand * Hand * Card
| NoHand of Hand
```
I can consume it with the deconstructing pattern matching syntax in C# like this:
```csharp
static ScoringList ScoreValue(ScorableHand s) =>
s switch {
ScorableHand.EntireHand (var h) =>
Seq.ToList(new [] {RankValue(h)}),
ScorableHand.SplitHand(var h, var k) =>
Seq.ToList(new [] {RankValue(h), RankValue(k)}),
ScorableHand.TwoHand (var bp, var sp,var k) =>
Seq.ToList(new [] {RankValue(bp), RankValue(sp), Rank(k)}),
ScorableHand.NoHand (var h) => Seq.ToList(Ranking(h)),
_ => throw new InvalidOperationException()
};
```

Instead of the existing way with `{Item#: var}`:

```csharp
static ScoringList ScoreValue(ScorableHand s) =>
s switch {
ScorableHand.EntireHand { Item: var h} =>
Seq.ToList(new [] {RankValue(h)}),
ScorableHand.SplitHand { Item1: var h, Item2: var k} =>
Seq.ToList(new [] {RankValue(h), RankValue(k)}),
ScorableHand.TwoHand {Item1: var bp, Item2: var sp, Item3: var k} =>
Seq.ToList(new [] {RankValue(bp), RankValue(sp), Rank(k)}),
ScorableHand.NoHand {Item: var h} => Seq.ToList(Ranking(h)),
_ => throw new InvalidOperationException()
};
```

## Pros and Cons

The advantages of making this adjustment to F# are that it would make it smoother for F# Discriminated Unions to be consumed from C#. And C# doesn't have their own Discriminated Unions.

The disadvantages of making this adjustment to F# would be if F# ever wanted to support the Deconstruct feature from C# it might cause complications for such an implementation.

## Extra information

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

C# lets you make Deconstruct methods by extension methods. So I was able to test this having this in a working sample.
https://github.com/jbtule/ConsumeFSharp-FromCSharp/pull/1

## 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

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