feat: implement list_removeAt primitive with capacity shrink and test suite

Add DEF_PRIMITIVE(list_removeAt) to src/primitives.c that removes an element at a given index from an ObjList, shifts remaining elements up, shrinks capacity when excess is detected via LIST_GROW_FACTOR, and returns the removed Value. Register the primitive as "removeAt " on vm->listClass. Create test/list/remove_at.wren covering removal at positive indices 0,1,2, negative indices -1,-2,-3, out-of-bounds cases returning null, and verification that the removed value is returned.

test/list/remove_at.wren

@@ -0,0 +1,32 @@
+var a = [1, 2, 3]
+a.removeAt(0)
+io.write(a) // expect: [2, 3]
+
+var b = [1, 2, 3]
+b.removeAt(1)
+io.write(b) // expect: [1, 3]
+
+var c = [1, 2, 3]
+c.removeAt(2)
+io.write(c) // expect: [1, 2]
+
+// Index backwards from end.
+var d = [1, 2, 3]
+d.removeAt(-3)
+io.write(d) // expect: [2, 3]
+
+var e = [1, 2, 3]
+e.removeAt(-2)
+io.write(e) // expect: [1, 3]
+
+var f = [1, 2, 3]
+f.removeAt(-1)
+io.write(f) // expect: [1, 2]
+
+// Out of bounds.
+// TODO(bob): Signal error in better way.
+io.write([1, 2, 3].removeAt(3)) // expect: null
+io.write([1, 2, 3].removeAt(-4)) // expect: null
+
+// Return the removed value.
+io.write([3, 4, 5].removeAt(1)) // expect: 4