Fix compatibility with swift-argument-parser 1.3.1

[liamnichols]

The latest version of swift-argument-parser has a fix that causes an incompatibility with the current Generate command definition:

• @Option- and @Argument-annotated optional properties that use a transform closure for parsing can now be declared without ambiguity. (Fix for @Option(transform:) with optional type apple/swift-argument-parser#619)

https://github.com/apple/swift-argument-parser/releases/tag/1.3.1

This causes the issue observed in #44.

The fix however is simple, we just need to explicitly annotate input and output as URL so that they are not interpreted as optional types when using 1.3.1.