Extensible Numeric Literals: Stage 1

Champion: @littledan
Repo: https://github.com/littledan/proposal-extensible-numeric-literals
Slides: https://docs.google.com/presentation/d/13Ej08CCqXGCTF46GabGdzBepWBgU5d70TtPPYFkkZcs/edit#slide=id.p
First presented at the Sept 2017 meeting

## Examples

```
1236536253453n BigInt (special case)
4525i Imaginary numbers
235435.461m IEEE 754-2008 64-bit decimal
300px CSS Typed OM
```

Start with the proposal repository and linked slides, then review the examples for BigInt, imaginary numbers, decimal literals, and CSS Typed OM. The issue does not name implementation files, tests, or a concrete completion criterion, so the work would first require defining the Stage 1 scope.

Written by the indexing model from the issue text.