Subscript operator for lists.

test/list_subscript.wren

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+// Returns elements.
+var list = ["a", "b", "c", "d"]
+io.write(list[0]) // expect: a
+io.write(list[1]) // expect: b
+io.write(list[2]) // expect: c
+io.write(list[3]) // expect: d
+
+// Allows indexing backwards from the end.
+io.write(list[-4]) // expect: a
+io.write(list[-3]) // expect: b
+io.write(list[-2]) // expect: c
+io.write(list[-1]) // expect: d
+
+// Handle out of bounds.
+// TODO(bob): Should halt the fiber or raise an error somehow.
+io.write(list[4])  // expect: null
+io.write(list[-5]) // expect: null
+
+// Handle wrong argument type.
+// TODO(bob): Should halt the fiber or raise an error somehow.
+io.write(list[true]) // expect: null
+
+// Handle non-integer index.
+// TODO(bob): Should halt the fiber or raise an error somehow.
+io.write(list[1.5]) // expect: null