Support custom field name for validation

Sometimes the convention from the server in json doesn't directly map to the styles on the front end. For example, data from the API may be in snake_case when the javascript is using camelCase. Since this library is already parsing and converting raw json to correctly formed typescript compatible types, it seems this would be a great layer to adapt to these differences.

For example, something like the following syntax would be ideal:

const userDecoder: Decoder<User> = object({
  firstName: string('first_name'),
  lastName: string('last_name'),
  username: string()
)};

where the the string decoder has an optional string parameter.

Start by locating the current string and object decoder entry points and inspect how raw JSON fields are mapped to TypeScript properties. Use the proposed firstName/first_name example to define the supported optional field-name syntax, then add coverage showing that snake_case input validates against camelCase fields while existing username behavior remains unchanged.