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.

[spytheman]

@kbkpbot I just want to thank you in advance, for your continuing work on this 🙇🏻 .

[JalonSolov]

Most of the failures appear to be "strings imported but never used"...

[kbkpbot]

Most of the failures appear to be "strings imported but never used"...

There are some bugs in the code, and need more fix.
for example, .str() will used some extra module, such as json2, that need to import.
I will fix this.

Currently, I am working on PR #26079, this will enable cast alias in .str() function to show correct alias information.