feat: add generator for meta boxes

[dustin-jw]

Description

This works sets up a generator for registering meta boxes on different post types. It only covers a basic text input, leaving more advanced implementations up to developers.

Closes #133

To Validate

1. Make sure all PR Checks have passed
2. Pull down this branch
3. Run npm start
4. Run npm run generate:meta-box and answer the questions, at least enabling the meta box for default post posts
5. Add or edit a post, confirming that the meta box shows up in the editor with the values you specified
6. Enter some data in the field, and save your changes, confirming that no errors are thrown and that the value persists when you reload the page
7. Add other meta boxes with different values for context/positioning/post types and confirm that they work as you would expect (more or less, context and priority seem more like suggestions than rules)

[robtarr]