feat: add list subscript setter and refactor parameter validation

Implement list subscript setter (`[ ]=`) native method in wren_core.c, enabling assignment to list elements via bracket syntax. Extract duplicate parameter count validation logic into a shared `validateNumParameters` function in wren_compiler.c, replacing inline checks in both `parameterList` and `methodCall`. Add comprehensive test suite covering basic assignment, return value semantics, negative indices, and error cases for out-of-range and excessive arguments.

test/list/subscript_setter.wren

@@ -0,0 +1,38 @@
+// Basic assignment.
+{
+  var list = [1, 2, 3]
+  list[0] = 5
+  list[1] = 6
+  list[2] = 7
+  io.write(list) // expect: [5, 6, 7]
+}
+
+// Returns right-hand side.
+{
+  var list = [1, 2, 3]
+  io.write(list[1] = 5) // expect: 5
+}
+
+// Negative indices.
+{
+  var list = [1, 2, 3]
+  list[-1] = 5
+  list[-2] = 6
+  list[-3] = 7
+  io.write(list) // expect: [7, 6, 5]
+}
+
+// TODO: Handle out of range.
+// TODO: Wrong non-number type.
+// TODO: Floating-point subscript.
+//{
+//  var list = [1, 2, 3, 4]
+//  list[4] = 1
+//}
+//
+//{
+//  var list = [1, 2, 3, 4]
+//  list[-5] = 1
+//}
+
+// TODO: Not in this dir, but need tests for subscript setter grammar.