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.

builtin/core.wren

@@ -1,4 +1,13 @@
 class Sequence {
+  contains(element) {
+    for (item in this) {
+      if (element == item) {
+        return true
+      }
+    }
+    return false
+  }
+
   count {
     var result = 0
     for (element in this) {
@@ -92,15 +101,6 @@ class List is Sequence {
     }
     return result
   }
-
-  contains(element) {
-    for (item in this) {
-      if (element == item) {
-        return true
-      }
-    }
-    return false
-  }
 }
 
 class Map {