Enforce better type checking

[samfry13]

This PR adds a few things related to typescript types:

• Adds prop types to any component that accepted props
• Updated the newsletters content collection variable name to match the directory name. They previously didn't match, which meant that the zod schema was never being run on the newsletter frontmatter.
• Adds tests to make sure newsletters are generated with the proper metadata
• Adds title check to newsletters to make sure they follow This Month in Ladybird - <month> <year>
• Adds a type check step to CI

[gmta]

@samfry13 Thanks for originally setting this up! I cleaned up your branch a bit and updated it for the latest changes.