This MR adds support for a couple of improvements in the existing
extensions and also backfills tests for serialization for all the
extensions.

Support added for:
* Multiline blockquotes and retaining it using source maps
* Maintaining start position of a numeric list
* Maintaining numeric list style: `1. ` vs `1) `
* ^ Same improvement in numeric task lists
* Serializing plain URLs as `<url>` instead of `[url](url)`

Changelog: added