Core utilities for foundry-compilers crates.

Provides shared error types, path utilities, and optional helpers used across the compiler pipeline.

• async: Adds async file I/O via tokio.
• hasher: Enables xxhash based hashing.
• regex: Enables regex-based utilities.
• svm-solc: Enables svm for managing solc installations.
• walkdir: Enables recursive directory walking.
• project-util: Utilities for creating and testing project workspaces.
• test-utils: Test helpers (tempdir creation, etc.).