Accept and ignore calendar annotation in Instant string

Parsing an Instant string already ignores the time zone name annotation if
one is present, and only takes the offset into account. It's inconsistent
to throw if a calendar annotation is present but not if a time zone name
annotation is present.

UPSTREAM_COMMIT=3cd96697efb2f03120bf0d4c5ad7d3b65d2e20d4

Author: ptomato

test/validStrings.mjs

@@ -360,7 +360,7 @@ const duration = seq(
   choice(durationDate, durationTime)
 );
 
-const instant = seq(date, [timeSpecSeparator], timeZoneOffsetRequired);
+const instant = seq(date, [timeSpecSeparator], timeZoneOffsetRequired, [calendar]);
 const zonedDateTime = seq(date, [timeSpecSeparator], timeZoneNameRequired, [calendar]);
 
 // goal elements