cgen: fix cross method call + method_call cleanup

[felipensp]

Fix #20012

🤖[deprecated] Generated by Copilot at 6243624

This pull request improves the C code generation for method calls in V. It refactors some functions, introduces a new function to resolve the receiver type name, and adds a new test case to cover a previously unhandled scenario. The changes aim to make the code more readable, maintainable, and correct.

🤖[deprecated] Generated by Copilot at 6243624

• Refactor C code generation for method calls (link, link, link, link)

• Extract resolve_receiver_type function to vlib/v/gen/c/assign.v to get receiver type and symbol from type table (link, link)
• Move method_call function to vlib/v/gen/c/assign.v to handle method calls in assignments and expressions (link)
• Add resolve_receiver_name function to vlib/v/gen/c/assign.v to get C name of receiver type for method calls (link)

• Add test case for cross method calls with maps and move method to vlib/v/tests/cross_method_call_test.v (link)