Error on misspelled property definition.
Error on misspelled property definition.

We allow "property" or "properties" / "invariant" or "invariants" in
their respective positions and error otherwise.

Tested by changing `@model (property conserves-mass)` to `@model
(propety conserves-mass)` in verified accounts. Saw this error message:

```
examples/verified-accounts/accounts.pact:48:11: could not parse (propety conserves-mass): expected "property" or "properties"
```

Fixes #141.

kadena-io/pact,kadena-io/pact,kadena-io/pact,kadena-io/pact