Clarify license conditions

Hi! I'm considering packaging this project for Debian, since it appears to be required for the `age` self-tests in v1.2.0. The alternative is to patch `age` to avoid using this package, but that doesn't sound ideal. However the license terms are not clear.

Could you clarify what license this project generally uses, and review each file if it is actually is consistent with that license? Or add per-file license statements. Or at least add some guidance on how license matters are dealt with in this project.

There are some license statements today, but they are a bit confusing. For example, there is a BSD-3-Clause statement in `age/internal/LICENSE` but the file `age/internal/bech32/bech32.go` has a Expat license.

Thanks,
Simon

Start by comparing the BSD-3-Clause statement in age/internal/LICENSE with the Expat license in age/internal/bech32/bech32.go, then review the project's existing license statements. Done means the project license approach is clearly documented and the relevant files have consistent statements or clear guidance on how licensing is handled.

Written by the indexing model from the issue text.