Write language specification

[mjp41]

### Tasks
- [ ] https://github.com/microsoft/verona/issues/656
- [ ] https://github.com/microsoft/verona/issues/657
- [ ] https://github.com/microsoft/verona/issues/658
- [ ] https://github.com/microsoft/verona/issues/659
- [ ] https://github.com/microsoft/verona/issues/660
- [ ] https://github.com/microsoft/verona/issues/661
- [ ] https://github.com/microsoft/verona/issues/662
- [ ] https://github.com/microsoft/verona/issues/663
- [ ] Write latter sections of Language spec