Licensing mismatch

Hi @appmattus,

I noticed while looking at the source that the https://github.com/appmattus/markdown-lint/tree/main/markdown/src/main/kotlin/com/appmattus/markdown/checkstyle package's code is under LGPL-2.1, which folk usually consider to clash with the overall Apache-2.0 licensing of the project. 

Was this intended?