Enforce `AttributeTargets` properly.

**I propose we** Enforce the correct use of `AttributeTargets` in the language and write a specification to document it.

**The existing way of approaching this problem in F# is**

In most cases just use interchangeably custom attributes with `AttributeTargets` and the compiler will not raise any errors.

```fsharp
open System

[]
type MethodOnlyAttribute() =
inherit System.Attribute()

[]
let someFunction () = "abc"

[] // Should error with: This attribute is not valid for use on this language element
let someValue = "abc"

[]
type FieldOnlyAttribute() =
inherit System.Attribute()

[] // Should error with: This attribute is not valid for use on this language element
let someFunction () = "abc"

[]
let someValue = "abc"
```

Note: The above snippet is just one of the many issues with `AttributeTargets`.

## Pros and Cons

**The advantages of making this adjustment to F# are**
- The compiler will raise errors when using an incorrect `AttributeTargets`
- We will have a language specification about where and how this `AttributeTargets` are meant to used.

**The disadvantages of making this adjustment to F# are**
- Any existing custom defined attribute that uses and incorrect `AttributeTarget` will get an error.

## Extra information

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

**Related suggestions:** (put links to related suggestions here)

## Affidavit (please submit!)

Please tick these items 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] This is a language change and not purely a tooling change (e.g. compiler bug, editor support, warning/error messages, new warning, non-breaking optimisation) belonging to [the compiler and tooling repository](https://github.com/dotnet/fsharp)
* [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
* [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

Please tick all that apply:
* [ ] 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 AttributeTargets examples in the issue and the existing F# language specification. Determine the language elements that must be checked and how the compiler should report invalid targets; done means the behavior is specified and incorrect uses produce errors. No files, tests, or compiler entry points are identified in the payload.

Written by the indexing model from the issue text.