fix: prevent duplicate keys by reusing tombstones in map probe sequence

Previously, when inserting a key that already existed in the map but was
located after a tombstone in the probe sequence, the insertion would stop
at the first tombstone and add the key there, resulting in duplicate
entries for the same key. This fix introduces a `findEntry` function that
tracks the first encountered tombstone during probing and reuses it for
insertion only when the key is not found elsewhere in the sequence.

test/core/map/reuse_tombstone.wren

@@ -0,0 +1,9 @@
+// Regression test for #373.
+var map = {}
+map[2] = "two"
+map[0] = "zero"
+map.remove(2)
+map[0] = "zero again"
+map.remove(0)
+
+System.print(map.containsKey(0)) // expect: false