gnomon-export: asymmetric warning APIs between iCal and JSCal exporters

[eikopf]

Description

`emit_icalendar` takes `&mut Vec` for warnings (used to report unrecognized non-extension fields). `emit_jscalendar` has no warnings mechanism at all — unknown fields are silently emitted as vendor properties.

This means callers get diagnostic feedback from iCal export but not JSCal export. Consider adding a warnings parameter to `emit_jscalendar` for consistency, or unifying the approach (e.g. a shared `ExportDiagnostics` type).