Allow to use subscript numbers in F# code

I sometimes work with scientific code and it's inconvenient in some cases. When I program a formula, it is very convenient to enter it as is. For example, there's a formula `Z₀ = E / H` (the first real example I found in my notes when performing calculations related to the [impedance of free space](https://en.wikipedia.org/wiki/Impedance_of_free_space)), which I'd like to see in my code as it looks here. But unfortunately, in F# I have to write it as `Z_0 = E / H`, since `Z₀` doesn't compile ([link to Sharplab](https://sharplab.io/#v2:EYLgtghglgdgNAGxAMwM5wC4iguATEAagB8B7ABwFMYACAZQE9UNKwBYAKAUoxoFEaAXhoBGAAyduvABJCaAVglceNAFqAAgjkCA9DWmcgA=)).

> ```
> error FS0010: Unexpected character '₀' in binding. Expected '=' or other token.
> ```

**I propose we** add a possibility to use subscript numbers in F# code, such as:
```fsharp
let E = 10
let H = 50

let Z₀ = E / H // this!
```

Notably, it's possible to use subscript characters such as `ₖₒₐₗₐ`, but not numbers.

**The existing way of approaching this problem in F# is** to use a notation of `Z_0` instead, which doesn't look that good.

## Pros and Cons

**The advantages of making this adjustment to F# are**: there will be easier to write and, more importantly, read code based on formulas in scientific calculations.

**The disadvantages of making this adjustment to F# are**: well, it'd be easier to abuse this feature and name certain symbols in
- either a confusing way
- or in a way that makes it harder to reuse the values in other languages (say, C# doesn't allow `Z₀` either).

Note that this is about subscript numbers for now, and not superscripts. Superscripts would be more confusing, since `E²` is easy to mix with `E ** 2`.

## Extra information

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

**Related suggestions:** none that I was able to find.

## 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

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 proposed F# syntax and the SharpLab example in the issue, including the contrast between subscript letters and numbers. Determine the language-design and compiler work needed to accept bindings such as Z₀, then validate the accepted and rejected identifier forms with compiler tests; done means the proposal is resolved and the behavior is specified and tested.

Written by the indexing model from the issue text.