[typescript] migrate node-fetch to undici

[davidgamero]

looking for input on viability of moving towards compatibility with native fetch api

native fetch is undici. using the direct 'undici' module allows more advanced use cases

node-fetch hasn't updated in 2 years

breaking internal changes include:

• RequestInit.agent -> RequestInit.dispatcher
• Upgrade types to node20
• node encode-decode test drops end-of-life versions 16 and 18. adds 22 and 24
• package.json types extension js->ts
  • "types": "./dist/index.d.js" -> "types": "./dist/index.d.ts"
  • spec indicates it should be index.d.ts handbook entry, but i'd like input on if this does get used
• added "module":"commonjs" for commonjs (es6 support off) generation to typescript
  • this will fundamentally change built output for existing packages, and im open to the option of just updating the encode/decode test to use ESM/ES6 instead of this change

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