Make Sharp an optional peer dependency

Currently the 'sharp' model is a direct dependency, but I believe it can/should be moved to optional peer dependencies. The module is only required when using ndarray-pixels in Node.js, and requires platform-specific binaries that can make life complicated, especially if another version of sharp is required by another dependency. If not a peer dependency then perhaps the version range could be loosened.

Start by checking the package manifest and the Node.js image-loading entry point to see how sharp is declared and loaded. Confirm that the dependency arrangement supports another sharp version and that browser usage remains unaffected; validate the result with the repository's existing checks.

Written by the indexing model from the issue text.