Reject unknown fields when parsing .yaml files

[byorgey]

Is your feature request related to a problem? Please describe.
I just spent a frustrating 20 minutes trying to figure out why my scenario didn't work the way I wanted, until I finally realized that the seed: field should be at the top level, rather than indented under world:. No error was generated because (1) the seed is optional, and (2) unknown fields are simply ignored.

Describe the solution you'd like
We should have a standard way to check for, and reject, unknown/misplaced/misspelled fields. For example, see this StackOverflow question: https://stackoverflow.com/questions/67375099/prevent-unknown-field-names-in-aeson-parsejson

[xsebek]

We could show a Warning on it and ask the player to confirm:

$ swarm --scenario MyScenario.yaml
WARNING: unrecognized field in scenario file (15:2):
- "gaol"
+ did you mean "goal"?
There was 1 warning generated for the scenario file "MyScenario.yaml"!
**Would you like to play it anyway? Type [y]es or [n]o:**
> no please just fix it for me
**Would you like to play it anyway? Type [y]es or [n]o:**
> n

We should check for such warnings in tests of course. 👍

[byorgey]

Well that is a lot fancier than what I was thinking, but sure, why not. 😄

[byorgey]

According to the link, for types where the FromJSON instance is derived generically, we can just do something like

instance FromJSON MyType where
  parseJSON = genericParseJSON $ defaultOptions
    { rejectUnknownFields = True }

For other types where we have a custom FromJSON instance, we'll have to do more manual checking. In the linked StackOverflow answer, this happens by checking the keys of the HashMap we get from aeson.

[kostmo]

This is now implemented: #1475 (comment)