Addition of a correct modulus operator

# Addition of a correct modulus operator

The current remainder operator `%` can be surprising in that the sign of the result follows the sign of the dividend, so does not output periodic results for a fixed divisor.
Changing the current behavior of `%` has been declined in https://github.com/fsharp/fslang-suggestions/issues/417, so another way forward would be to add a new operator,
which, like a true modulus operator, [would output positive results regardless of its inputs](https://math.stackexchange.com/q/2686378).
By always outputting positive results, we would achieve predictability and ease of use for periodic computations.

From F#'s OCaml roots, `mod` is keyword for an infix operator already, and is currently undefined in FSharp.Core:
```fs
let inline (mod) D d =
let r = D % d
if r >= LanguagePrimitives.GenericZero then r
elif d >= LanguagePrimitives.GenericZero then r + d
else r - d
// Usage: -5 mod 3
```

The existing way of approaching this problem in F# is to include such a definition everytime we use the operator, or bite ourselves when trying to use `%` on negative dividends.

## Pros and Cons

The advantages of making this adjustment to F# are
1. Raise awareness of the true modulus operator
2. Less duplicated code across projects
3. Assigning novel uses to existing available operators

The disadvantages of making this adjustment to F# are that the presence of both `%` and `mod` can be confusing. However, upon realisation that `%` stands for the remainder operation instead of modulus, and recognising the difference between the two operations, this can be solved.

## Extra information

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

Related suggestions:
- https://github.com/fsharp/fslang-suggestions/issues/417

## 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

## For Readers

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