Add ability to ignore build-dependencies when checking for licenses

[acpiccolo]

This PR (AI generated) for issue #744 adds a new include-build configuration option to the [licenses] section, allowing users to opt out of checking build-dependencies for license checks.

What changed:

src/licenses/cfg.rs: added include_build (TOML key: include-build) with default true and validation.
src/licenses/gather.rs: respect include-build (in combination with include-dev) when selecting which crates to check for licenses.
docs/src/checks/licenses/cfg.md: documented include-build.
Added a unit test and updated snapshots for deserialization.

Notes:

Default behavior preserved: build-dependencies are included by default to avoid breaking existing usage.

Please review and comment.

Resolves: #744