feat: allow file paths as input

[HakonHarnes]

Related issue

Closes #40.

Summary of changes

• Moved main logic from init.lua to paste.lua
• Restructured the code so it can handle file paths and URLs as inputs
• Implemented logic for embedding local images (via file paths) as Base64
• Renamed function from pasteImage to paste_image, but kept pasteImage to ensure backwards compatibility
• Allow file paths or URLs to be passed directly to the paste_image function via the API
• Simplified the util.execute function
• Updated tests
• Updated docs