Decimal, nativeint and unativeint literals

# Decimal, nativeint and unativeint literals

I propose we allow decimal, nativeint and unativeint literals.

The existing way of approaching this problem in F# is (none).

C# 9 now has [native-sized integers](https://github.com/dotnet/csharplang/blob/b75760a72480d4214bf68d3150a2ff649c80e95e/proposals/NativeInt.md).
[Together with decimal constants, C# 9 can now define three types of literals that F# cannot](https://sharplab.io/#v2:EYLgZgpghgLgrgJwgZwLQDlYEsBuECSAdjAD4ACATAAQDCVA3gLABQVbVAxgPaHIxUATCBywBbKABsqAEWFjJVALxUAzAG4W7Tjz5VCWYlXQH+ygGwbW7br36E4Jow8PKAnJYC+QA===). To interface with them, we must be able to interpret and define decimal, nativeint and unativeint literals.

```fs
let [] Decimal = 3m
let [] NativeInt = 3n
let [] UNativeInt = 3un
match 3n with NativeInt -> true | _ -> false // true
```

## Pros and Cons

The advantages of making this adjustment to F# are
1. Consistency between numeric types - all other numeric types can be defined as literals. Why not these three?
2. Increased interoperability with C#
3. More code reuse - more chances to refactor constant patterns in `match's into literals that can be changed easily

The disadvantages of making this adjustment to F# are none that I can think of.

## Extra information

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

Related suggestions:
https://github.com/fsharp/fslang-suggestions/issues/562

## 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
* [x] I or my company would be willing to help implement and/or test this

No source files, tests, or entry points are named. Start with the proposed literal examples and related suggestion #562, then review the referenced C# native-sized integer proposal; done means an agreed implementation path covering 3m, 3n, 3un, Literal declarations, and match behavior.

Written by the indexing model from the issue text.