add initial snapshot tests

[vezwork]

Description

• Adds a readOrCreateSnapshot helper function that makes snapshot tests convenient to create:
  • If you don't already have a snapshot, it creates one with the passed contents and returns the contents
  • If you do already have a snapshot, it reads it and returns the contents
• Adds two roundtripping snapshot tests based on Quarto documents from https://github.com/quarto-dev/quarto-markdown-test-corpus
  • valid-basics.qmd
  • invalid.qmd

For fun, here's a screenshot of the diff between valid-basics.qmd before and after roundtrip

I got this in VSCode by right clicking on valid-basics.qmd, then clicking "select for compare", then right clicking generated_snapshots/roundtripped-valid-basics.qmd, then clicking "compare with selected".

[juliasilge]

This works great for me locally as well!