Introduce CancelToken and CancelTokenCallback types to replace Axios imports

[pascalbaljet]

This PR unifies the types around cancel tokens. Previously, we had a mixed bag of implementations.

import type { CancelTokenSource } from 'axios'

type VisitCallbacks = {
  // Core adapter...
  onCancelToken: { ({ cancel }: { cancel: VoidFunction }): void },

  // Svelte adapter...
  oncanceltoken?: (event: CustomEvent<CancelTokenSource>) => void
}

// Vue adapter...
defineProps({
  onCancelToken: {
    type: Function as PropType<(cancelToken: import('axios').CancelTokenSource) => void>,
  }
})

We now have unified internal types in the core package. These are used across all adapters without any dependency on Axios types.

This was originally planned in PR #2549, but I decided to split it up into multiple PRs for clarity.