feat: move contains method from List to Sequence and add tests for List and Range

Add a generic `contains` implementation on the `Sequence` base class, removing the duplicate method from `List`. This allows all sequence types (including Range) to use the method without per-class redefinition. Include new test files for `List.contains` and `Range.contains` covering ordered, backwards, and exclusive ranges.

test/core/list/contains.wren

@@ -0,0 +1,6 @@
+var list = [1, 2, 3, 4, "foo"]
+
+IO.print(list.contains(2))      // expect: true
+IO.print(list.contains(5))      // expect: false
+IO.print(list.contains("foo"))  // expect: true
+IO.print(list.contains("bar"))  // expect: false

test/core/range/contains.wren

@@ -0,0 +1,23 @@
+// Ordered range.
+IO.print((2..5).contains(1)) // expect: false
+IO.print((2..5).contains(2)) // expect: true
+IO.print((2..5).contains(5)) // expect: true
+IO.print((2..5).contains(6)) // expect: false
+
+// Backwards range.
+IO.print((5..2).contains(1)) // expect: false
+IO.print((5..2).contains(2)) // expect: true
+IO.print((5..2).contains(5)) // expect: true
+IO.print((5..2).contains(6)) // expect: false
+
+// Exclusive ordered range.
+IO.print((2...5).contains(1)) // expect: false
+IO.print((2...5).contains(2)) // expect: true
+IO.print((2...5).contains(5)) // expect: false
+IO.print((2...5).contains(6)) // expect: false
+
+// Exclusive backwards range.
+IO.print((5...2).contains(1)) // expect: false
+IO.print((5...2).contains(2)) // expect: false
+IO.print((5...2).contains(5)) // expect: true
+IO.print((5...2).contains(6)) // expect: false