[Feature] tree sitter markdown scan

[mwittie]

Describe your changes

• Scan markdown files using tree-sitter into an array of content blocks. This is the first step in moving from input.md to output.md through copying and executing content cells.
• add go fmt and go vet to CI rules

Notes for reviewers

Checklist before requesting a review

If any of these checks are missing, please provide an explanation.

• I have updated README files, if applicable.
• This PR is small, otherwise explain why.