feat: split nights off an accommodation booking as separate periods

addresses #869f6z36v

src/Enum/Groups/AccommodationBookingPeriodPosition.php

@@ -0,0 +1,14 @@
+<?php
+
+namespace App\Enum\Groups;
+
+/**
+ * Which edge of the stay a split-off period sits at. Periods only ever come off the start or
+ * the end, so the main period — the one the booking's own parameters price — always stays a
+ * single contiguous range between them.
+ */
+enum AccommodationBookingPeriodPosition: string
+{
+    case Leading = 'leading';
+    case Trailing = 'trailing';
+}