adapt new method semantic

[Guest0x0]

Adapt the semantic change in moonbitlang/core#1472:

• update documentation of method
• fix warnings in test sources

[peter-jerry-ye-code-review]

‼️ This code review is generated by a bot. Please verify the content before trusting it.

After analyzing the git diff, here are the key problems I've identified:

1. Inconsistent Method Invocation: Throughout the code, there are changes from List::of to @immut/list.of. This indicates that the code was using the wrong namespace/package for list operations. The changes make it more explicit by using the qualified path @immut/list.of.

2. Missing Namespace Qualification: In the operator overloading section, there's a change from push(5) to Array::push(5), suggesting that the original code was missing proper namespace qualification for the Array methods. This could have led to ambiguity or incorrect method resolution.

3. Documentation Inconsistency: In the methods documentation (methods.md), there are structural changes in how method definitions are explained. The original documentation was less clear about the two different ways to define methods (fn method_name(self: T, ..) vs fn T::method_name(..)). The updated version makes this distinction more explicit and provides clearer guidelines on when to use each approach.

These changes generally improve code clarity and reduce potential ambiguity in method resolution and namespace usage.