[possible regression] Updating to v0.18.0 no longer throws an error when parsing MDC with invalid formatting

[adamdehaven]

The changes in v0.18.0 (likely related to remark-mdc) now prevent console warnings and invalid MDC from throwing an error via the parseMarkdown utility function.

Before this update, the following MDC would cause the parseMarkdown function to throw an error that could be utilized to surface an error to a user about invalid indentation, etc.

---
title: "My page title"
---
# My page title

::div
  :::div
  ---
  first-prop: "ok"
invalid-prop: "indentation"
  ---
  :::
::