Fully switch to via-IR codegen

### What

Switch the default code generation backend.

### Why

Switching to this will help alleviate stack-too-deep errors, it will allow better optimizations thanks to the YUL optimizer.
The compiler output code will be more human-readable.
The codegen is more uniform and, therefore, less error-prone, reducing the risk of compiler bugs.

### How

Informing Tooling teams to prepare to make this switch.
We won't remove the old code generation pipeline immediately.
If we don't have any significant negative feedback, we'll remove it 2 quarters later.

#### Tasks

- [ ] ...

### Notes

### Out of scope

- TBD

---
### Resources

1. [Roadmap items](https://github.com/orgs/ethereum/projects/26/views/1?filterQuery=label%3A%22via-IR%22)

No source files, tests, or compiler entry points are identified in the issue. Start by reviewing the linked via-IR roadmap items and the current code-generation pipeline, then coordinate with the tooling teams. Done means the default backend has switched while the old pipeline remains available for the stated feedback period.

Written by the indexing model from the issue text.