Public [<Literal>]s as static members

I propose we allow public [\]s as static members.

The existing way of approaching this problem in F# is forcing to define the [\] in global level or in a module.

e.g.
```fs
[] type UserID = private UserID of int
with [] static member Maximum = 9999 //error currently, should allow

type ``THREE!!!!!!!!!``() =
[]
let privateLiteral = 3 //currently allowed
[]
static member PublicLiteral = 3 //error currently, should allow
```

## Pros and Cons

The advantages of making this adjustment to F# are
1. one more parity with C#
2. make literals more first class

The disadvantage of making this adjustment to F# is implementation effort required.

## Extra information

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

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