Default to native fetch not @web-std/fetch

[alanshaw]

Node 18 has a global fetch implementation so @web-std/fetch should not be needed for much longer. Please can we be consistent with ucanto and detect and use the native fetch implementation rather than depending on this module. Library consumers can install their own fetch implementation if they want to use this in an environment that does not support native fetch.

Note: need to globalThis.fetch.bind(globalThis) to avoid TypeError: 'fetch' called on an object that does not implement interface Window. e.g. storacha/ucanto#88