Add support for `duration: "0"` (without units)

[zachleat]

• duration: "0"
• maybe also duration: 0 (number)
• maybe also any Number? (guessing this should be seconds, as that’s the smallest shorthand unit)
• maybe also duration: Infinity (same as "*") this was shipped with 9c54ad0 (v2.0.1+)

See also https://www.11ty.dev/docs/plugins/fetch/#change-the-cache-duration