Plugin: PDF

**Is your feature request related to a problem? Please describe.**
I'm currently trying to save the image as PDF for the needs of my project.

**Describe the solution you'd like**
I would want to easily save the image as PDF. Like: "image.write('picture.pdf')

**Describe alternatives you've considered**
Maybe needs another library to convert the buffer into a PDF.

**Additional context**
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No file, test, or entry point is named in the request. Start by locating Jimp's plugin system and the image.write entry point, then assess the proposed PDF conversion dependency; done means an image can be written as a PDF through the requested API with coverage for that behavior.

Written by the indexing model from the issue text.