Podcastindex JSON Chapter support

[mpgirro]

The Podcastindex standard provides chapter marks as an external file in JSON Chapter Format. In our implementation of the podcast: namespace (phase 1), we've skipped JSON chapter parsing and writing so far. For a possible future support, we should consider how this can blend in nicely into our existing API.

Supporting this will also impact a future #63 Multiplatform migration, depending on the JSON API we are going to use.

[mpgirro]

My initial proposal is to provide PodcastindexJsonChapterParser/PodcastindexJsonChapterWriter objects to be consistent with our existing PodcastRssParser/PodcastRssWriter. The API can also look similar.

[rock3r]

If we use kotlinx.serialization, that'll make our JSON implementation Multiplatform-compatible, so in that perspective I think it's worth considering it. Not sure if we can write an XML API wrapper for kotlinx.serialization, but that may make #63 easier to get to in the future.