chore: migrate to typescript

[okxiaoliang4]

migrate to typescript.

welcome to commit and review.

TODO:

• [ ] adapters
  • [ ] adapters/http
  • [ ] adapters/index
  • [ ] adapters/xhr
• [ ] cancel
  • [x] cancel/CanceledError
  • [ ] cancel/CancelToken
  • [ ] cancel/isCancel
• [ ] core
  • [ ] core/Axios
  • [x] core/AxiosError
  • [ ] core/AxiosHeaders
  • [ ] core/buildFullPath
  • [ ] core/dispatchRequest
  • [x] core/InterceptorManager
  • [ ] core/mergeConfig
  • [x] core/settle
  • [ ] core/transformData
• [ ] defaults
  • [ ] defaults/index
  • [x] defaults/transitional
• [x] env
  • [x] env/classes/FormData
  • [x] env/data
• [ ] helpers
  • [ ] helplers/AxiosTransformStream
  • [x] helplers/AxiosURLSearchParams
  • [x] helplers/bind
  • [ ] helplers/buildURL
  • [x] helplers/combineURLs
  • [x] helplers/cookies
  • [x] helplers/deprecatedMethod
  • [ ] helplers/formDataToJSON
  • [x] helpers/fromDataURI
  • [x] helpers/isAbsoluteURL
  • [x] helpers/isAxiosError
  • [x] helpers/isURLSameOrigin
  • [x] helpers/null
  • [x] helpers/parseHeaders
  • [x] helpers/parseProtocol
  • [x] helpers/speedometer
  • [x] helpers/spread
  • [x] helpers/throttle
  • [ ] helpers/toFormData
  • [ ] helpers/toURLEncodedForm
  • [ ] helpers/validator
• [x] platform
  • [x] platform/browser
  • [x] platform/node
  • [x] platform/index
• [ ] axios
• [x] utils

[amitsainii]

I'd love to help here if needed. I explored the axios codebase yesterday and I have opened a PR for a type fix today and also raised a type issue mentioning the possible solutions as well. If given the opportunity I'd love to contribute more frequently to this awesome project that I've been using for long without giving back to the community. This is just a request, I totally understand if this isn't feasible for some reason 👍