Extend custom numeric types to support floating point literals

Currently you can define custom numeric types for integer values.
It would be nice to be able to handle floating point values too, e.g.

```fsharp
type complex =
| Complex of double * double

type imaginery =
| Imaginery of double
static member (+) (lhs:double,Imaginery(rhs)) = Complex(lhs,rhs)

module NumericLiteralI =
let FromZero () = Imaginery 0.0
let FromOne () = Imaginery 1.0
let FromInt32 (x) = Imaginery (double x)
let FromInt64 (x:int64) = Imaginery (double x)
let FromDouble (x:float) = Imaginery x // extension
let polar = 1.5 + 2.5I
```

**[Original UserVoice Submission](https://fslang.uservoice.com/forums/245727-f-language/suggestions/8140617)**

No implementation files, tests, or entry points are named. Start by reviewing issue #445, its F# numeric-literal example, and the linked original UserVoice submission. Done means the requested floating-point literal support has an agreed scope and is validated by appropriate compiler tests.

Written by the indexing model from the issue text.