Simple typo-detection by matching MissingField with ExtraField

[MathiasPius]

Misspelling a field in a strictly typed object presently results in a SchemaError::Multiple(FieldMissing, ExtraField) error. It would be cool to do some simple levenshtein distance on these two field names to suggest a typo was made if the similarity is high.

SchemaError { 
  kind: Multiple { 
  errors: [
    SchemaError { 
      kind: FieldMissing { field: "not" }, 
    }, 
    SchemaError { 
      kind: ExtraField { field: "note" },
    }
  ]
}

Would thereby be transformed into:

SchemaError { 
  kind: Typo { 
    extected: "not",
    found: "note",
  }
}

[MathiasPius]

This should probably be implemented in the CLI-tool instead of in the library itself, since there's no reason to increase the possible number of error cases to handle, and it is mostly an aesthetic improvement.