JSDoc
Provide a synchronous function to coerce or transform the value(s) given on the command line for `key`.
The coercion function should accept one argument, representing the parsed value from the command line, and should return a new value or throw an error.
The returned value will be used as the value for `key` (or one of its aliases) in `argv`.
If the function throws, the error will be treated as a validation failure, delegating to either a custom `.fail()` handler or printing the error message in the console.
Coercion will be applied to a value after all other modifications, such as `.normalize()`.
Optionally `.coerce()` can take an object that maps several keys to their respective coercion function.
You can also map the same function to several keys at one time. Just pass an array of keys as the first argument to `.coerce()`.
If you are using dot-notion or arrays, .e.g., `user.email` and `user.password`, coercion will be applied to the final object that has been parsed