feat: add derive command

[jackchuka]

This pull request introduces a new "derive" command to the CLI, allowing users to infer a schema from an existing Markdown document. The schema inference logic is implemented in a new package, and comprehensive tests are included to ensure correct behavior, including handling of front matter and heading hierarchies.

New CLI command:

• Added a derive subcommand (NewDeriveCmd) to the CLI, which analyzes a Markdown file and generates a schema reflecting its heading hierarchy. The command supports outputting the schema to a file or stdout. [1] [2]

Schema inference logic:

• Implemented the infer package with a FromDocument function that builds a schema from a parsed Markdown document, correctly handling heading levels and skipping front matter.

Testing:

• Added tests for the inference logic, covering correct schema construction, error handling for documents without headings, and proper skipping of front matter.