Add containsKey() to Map, and validate key types.

test/map/contains_key.wren

@@ -0,0 +1,11 @@
+var map = {
+  "one": 1,
+  "two": 2,
+  "three": 3
+}
+
+IO.print(map.containsKey("one")) // expect: true
+IO.print(map.containsKey("two")) // expect: true
+IO.print(map.containsKey("three")) // expect: true
+IO.print(map.containsKey("four")) // expect: false
+IO.print(map.containsKey("five")) // expect: false