Make @napi-rs/canvas an optional dependency for text-only use cases

[kobbikobb]

Hi! Thanks for maintaining pdf-parse.

We use pdf-parse in a Node service that runs in Docker. We only call getText() for text extraction, never render to images. But @napi-rs/canvas is a hard dependency, which adds ~48MB of native binaries to our container.

Would you consider making @napi-rs/canvas an optional/peer dependency? Text extraction would work without it; rendering APIs would throw a clear error if it's not installed.

Quick options:

1. optionalDependencies + runtime check in render code paths
2. peerDependenciesMeta.optional: true
3. Split exports — e.g. pdf-parse/text (no canvas) vs pdf-parse (full)

Happy to send a PR if you're open to any of these — let me know which direction you'd prefer.

Either way, thanks for the library!