fix: add isCancel type assert

[chenjigeng]

add isCancel type assert

[jasonsaayman]

Closing as bool is the correct type in this case

[chenjigeng]

By isCancel type assert, when the function result is true, the next context typescript will treat the value is Cancel type. It will help developer to write code with code lint @jasonsaayman

[geekact]

The case when error has type AxiosError | Cancel, when we call axios.isCancel(err), the error value will change to type never

function test(err: AxiosError | Cancel) {
  if (axios.isCancel(err)) {
    err // AxiosError | Cancel
  } else {
    err // never
  }
}

I believe this's a bug.

[geekact]

My solution is remove Cancel interface and extend CanceledError

export class CanceledError<T = any> extends AxiosError<T> {
+ private __CANCEL__: true;
}

- interface Cancel {
-   message?: string;
- }