Fixes a bug, where empty lines from Gleam snippets were not used with checks. Also improves documentation.

This is a mostly backwards compatible release, as the only breaking change is adding new error variants.

Deprecated

• file was deprecated for clarity reasons. Use document instead.

Added

• Adds support for replacing snippets in code comments, using comments_in instead of document.
• Adds support using functions, function bodies, or type definitions as content to be checked or updated.

This is a complete rewrite. Instead of checking or running gleam code via checkmark, checkmark can now either check that a snippet matches the content in a file, or update the markdown file with the content.

Dependencies have also been simplified, and the parsing of the markdown file no longer uses the package it used to, as it had some issues in it, and doesn't seem to be actively maintained.