Refactor lowering from MIR to AIR

We should make the `translate_from_mir.rs` file a bit more readable - e.g., by splitting out some large `match` branches into helper functions and generally, maybe adding submodules etc.

_Originally posted by @bobbinth in https://github.com/0xMiden/air-script/pull/439#discussion_r2284332363_

The work is in translate_from_mir.rs. Start by reading the file to understand the MIR-to-AIR lowering logic, identify large match branches that can be extracted. Look for existing helper functions or modules in the codebase to follow patterns. The goal is to improve readability without changing functionality, so ensure tests pass after refactoring.

Vom Indexierungsmodell aus dem Issue-Text verfasst.