feat: add build info

[josephearl]

Adds a build output property with the build information. Fixes #10 and also #11.

Should extra be renamed to prerelease for consistency with SemVer terminology? (or perhaps have both for backwards compatibility?)

[matt-usurp]

Hey, thanks for this.

You know I wasn't aware of the "build" information, I only added the "extra" stuff because I wasn't aware if it was supported, but if its part of the semantic versioning specification (which it is, and the package supports) then I am happy with it. Appreciate the extra tests too. This makes me realise I should have some documentation for building the action (which I see you have done, so thanks too)

Should extra be renamed to prerelease for consistency with SemVer terminology? (or perhaps have both for backwards compatibility?)

I agree, if there is terminology here that makes more sense and follows the specification then I am all for it. So preferably the latter with it being replaced in the README and just marked as to be removed in the code for >=v3 with a comment.

[josephearl]

Have updated the PR to add prerelease as an output and replaced extra with prerelease in the documentation as suggested.

[matt-usurp]

Thanks!

[matt-usurp]

Release https://github.com/matt-usurp/validate-semver/releases/tag/v2.1.0 and also on v2 branch