Refactor: Move Ifs, Ranges, and Class from yash-semantics to yash-env

[Copilot]

This PR refactors the field splitting components by moving them from yash_semantics::expansion::split to yash_env::expansion::split, making these utilities available to other parts of the codebase without depending on the heavier yash-semantics crate.

What was moved

The following items have been relocated to yash-env:

New expansion module in yash-env

• expansion::attr

  • AttrChar: Character with attributes describing its origin
  • Origin: Category of syntactic elements from which expansion originates

• expansion::attr_strip

  • Strip trait for performing attribute stripping
  • Iter: Iterator wrapper for attribute stripping
  • Implementations for AttrChar and iterators

• expansion::split

  • Class: Type of characters that affect field splitting
  • Ifs: Collection of input field separator characters
  • Ranges: Iterator that yields index ranges of separated fields

Backward compatibility

All moved items are re-exported from their original locations in yash-semantics, ensuring complete backward compatibility. Existing code using these types through yash-semantics will continue to work without any changes:

// Both of these continue to work:
use yash_semantics::expansion::split::Ifs;  // ✓ Still works
use yash_env::expansion::split::Ifs;         // ✓ New location

Rationale

Moving these components to yash-env provides several benefits:

1. Better separation of concerns: Field splitting utilities are now in the environment layer where they logically belong
2. Reduced coupling: Other components can use these utilities without depending on yash-semantics
3. Improved modularity: The dependency graph is cleaner with utilities at the appropriate abstraction level

Version updates

• yash-env: 0.9.2 → 0.10.0 (minor bump for new public API additions)
• yash-semantics: 0.10.1 → 0.10.2 (patch bump for internal refactoring with re-exports)

Testing

• All existing tests pass without modification
• Clean build from scratch successful
• Full CI validation with ./check.sh passed
• Backward compatibility verified

Original prompt

Move Ifs, Ranges, and Class from yash_semantics::expansion::split to yash_env::expansion::split. Also move items that are depended on by them. This is a pure refactoring: no behavioral change must be made. After moving, the moved items should be re-exported in the original location to keep backward compatibility of the yash_semantics API.

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[magicant]

Absorbed in #639