utility: document bundling

[karenetheridge]

• method on JSM::Document::OpenAPI to combine two documents together: detect duplicate paths, components etc (with options to error, keep-first, discard-first, resolve duplicates by renaming (?))

[karenetheridge]

Note this is orthogonal to adding support for multiple documents (#73) which would require supporting $refs to external documents (and being able to hold multiple documents at once in an OpenAPI::Modern object).