chore: include licenses in published crates

[tisonkun]

Summary

• Include the Apache 2.0 license text in the exn crate package.
• Include the Apache 2.0 license text in the exn-anyhow crate package.
• Keep the SPDX license = "Apache-2.0" metadata without adding the redundant license-file field and its Cargo warning.

Validation

• cargo package --allow-dirty -p exn
• cargo package --allow-dirty -p exn-anyhow
• Byte-for-byte comparison of each packaged LICENSE with the repository root license
• cargo x lint
• cargo x test --no-capture