Extensible viewers with lazy loading

[mk]

This adds the viewers to Clerk itself, instead of just using the nextjournal viewers. These viewers come with recursive lazy loading for Clojure collections and strings.

Compared the Nextjournal data viewers, which were inspired by the ones found in the browser devtools, these new viewers show more data of nested data structures without the need for expansion and should feel much more familiar to Clojurians.

The viewer system is open and allows users to override them on three levels:

1. root
2. per-namespace
3. per-form