[major] add subcommand for pdf modification (pdf input support)

[zod]

Overview

Refactor the CLI tool to add a subcommand for modifying / restyling PDF documents. In other words, support pdf as input for modification.

Thoughts

• This will require a system, which I would like to add already, for stylization of the output document via CLI flags & potentially a configuration file.
• This addition is potentially antithetical to the original purpose of the application, and could potentially change the CLI workflow and/or add significant code to the codebase, so it might be worth maintaining a version which maintains the current usage subset.

default functionality should stay the same, but also have a subcommand associated with it probably for documentation / design consistency purposes? maybe convert)

Potential Implementation

• Add CLI subcommand modify for modification of pdf files
• Add CLI subcommand convert which encapsulates the current functionality of the application.
• Main application when used from CLI should assume convert subcommand