Add number of sheets as a unit for number of parts

src/enums/MDCountPartsUnit.php

@@ -12,6 +12,7 @@
 enum MDCountPartsUnit implements MDValueEnumInterface, JsonSerializable, MDMeasurementUnitInterface {
 
     case parts;
+    case sheets;
     case pages;
 
     /**
@@ -26,6 +27,7 @@ public static function fromString(string $input):MDCountPartsUnit {
         return match($input) {
             'parts' => self::parts,
             'pages' => self::pages,
+            'sheets' => self::sheets,
             default => throw new MDpageParameterNotFromListException("Unknown parts counting unit"),
         };
 
@@ -43,6 +45,7 @@ public static function fromInt(int $input):MDCountPartsUnit {
         return match($input) {
             1 => self::parts,
             2 => self::pages,
+            3 => self::sheets,
             default => throw new MDpageParameterNotFromListException("Unknown parts counting unit"),
         };
 
@@ -80,6 +83,7 @@ public function toInt():int {
         return match($this) {
             self::parts => 1,
             self::pages => 2,
+            self::sheets => 3,
             # default => throw new MDpageParameterNotFromListException("Unknown measurement type"),
         };