Revert "Revert "Introduce Dateish.days-in-year method" (#5085)"

This reverts commit 4d43bbf.

src/core.c/Dateish.pm6

@@ -23,10 +23,23 @@ my role Dateish {
         nqp::atpos_i($days-in-month,$month) ||
           ($month == 2 ?? 28 + IS-LEAP-YEAR($year) !! Nil );
     }
-    method days-in-month(Dateish:D: --> Int:D) {
+
+    proto method days-in-month(|) {*}
+    multi method days-in-month(Dateish: Int:D $year, Int:D $month --> Int:D) {
+        self!DAYS-IN-MONTH($year,$month)
+    }
+    multi method days-in-month(Dateish:D: --> Int:D) {
         self!DAYS-IN-MONTH($!year,$!month)
     }
 
+    proto method days-in-year(|) {*}
+    multi method days-in-year(Dateish: Int:D $year --> Int:D) {
+        365 + IS-LEAP-YEAR($year)
+    }
+    multi method days-in-year(Dateish:D: --> Int:D) {
+        365 + IS-LEAP-YEAR($!year)
+    }
+
     method !year-Str(--> Str:D) {
         sprintf 0 <= $!year <= 9999 ?? '%04d' !! '%+05d', $!year;
     }