Suggestion: Add ways to disable not typed interpolated strings

I propose we have a way to improve interpolated string.

Currently with interpolated strings can be done with or without the type identifier ( `%s` for example ).
It can be harmful when we do some code refactoring, not-typed interpolated strings can produce unexpected results.

For example :

``` F#
let hello = "Hello World"
printf $"{hello}" // ✅ Builds fine
printf $"%s{hello}" // ✅ Builds fine, same as `printf "%s" hello`
```

After some code refactoring we could be in this situation :

``` F#
type HelloWorld = HelloWorld of string
let hello = HelloWorld "Hello World"
printf $"{hello}" // ❌ Builds fine, prints `HelloWorld "Hello World"`
printf $"%s{hello}" // ✅ Does not build ( Error FS0001 : This expression was expected to have type 'string' but here has type 'HelloWorld' )
```

I would suggest adding a warning when using not-typed interpolated strings. And an optionnal way to force the usage of typed interpolated string for a project (WarningsAsErrors ?).

## Pros and Cons

**The advantages of making this adjustment to F# are** :

- More safety when using interpolated strings
- Less errors when refactoring code
- Errors detected at compile time

**The disadvantages of making this adjustment to F# are** :

- It can be a pain to add the type identifier to all interpolated strings
- Warnings can be annoying when using not-typed interpolated strings

## Extra information

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

## 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.

No files, tests, or compiler entry points are named. Start by reviewing how F# interpolated strings are specified and how compiler warnings and project-level warning settings are defined; done means reaching agreement on the warning behavior and an opt-in way to require typed interpolations, with corresponding implementation and tests identified.

Written by the indexing model from the issue text.