Add support for AsyncApi 2.1

[char0n]

Release notes are here: https://deploy-preview-260--asyncapi-website.netlify.app/blog/release-notes-2.1.0

According the the spec, section AsyncAPI Version String, the versioning of major.minor will be forward compatible. We need to rename AsyncApi namespace to apidom-ns-asyncapi-2 to reflect this and support 2.1.0 version of specification as well within that same namespace. Adapter names needs to be changes as well. WebPack and Babel configuration needs to be aligned (ApiDOM Playground).

[char0n]

Note: https://deploy-preview-260--asyncapi-website.netlify.app/blog/release-notes-2.1.0 is just a tip of the iceberg. Diff of v2.0.0 and v2.1.0 must be inspected carefully.