builder,checker,ast: move auto str() before backend gen

[kbkpbot]

Feature request by #25826

This PR aims to decouple the auto_str() function from individual backend generators and move it to the frontend. This way, each backend can remove its own auto_str() implementation.
The general approach is to introduce a new phase, gen_auto_fn() in the builder—after the checker and before code generation. This phase leverages the results from the checker and markused to identify which types require the auto_str() function. It then manually creates these virtual functions, processes them using parser.parse_text(), and performs checks via check(). The output is subsequently passed to the respective backends for code generation.

Currently, only the generation of auto_str() for structs is supported. Future extensions may include support for other str() functions, with the ultimate goal of fully migrating this functionality to the frontend.