fix: Handle missing fetch implementation

[plustokens]

There is a lake of information in replicate documentation regarding nodejs integration:
https://replicate.com/docs/get-started/nodejs

This fix will help to solve "ReferenceError: fetch is not defined" by giving user a way to provide fetch implementation in options object.

[mattt]

Thanks, @plustokens. This matches the documented behavior, where the initializer has a configurable fetch option.

[harriswong]

A quick work around that worked for me is to use node v18. ie. nvm install 18.

[mattt]

A quick work around that worked for me is to use node v18. ie. nvm install 18.

@harriswong You are correct that Node.js 18 includes a native fetch function. There are other reasons that someone might want to configure which fetch to use. Testing is one example. Another is to wrap fetch and add custom headers / caching / retry behavior.