Export RequestConfig and related types

[simplesmiler]

Use case:

// No public type for this
const config = {
  params: { ... },
  axios: { ... },
};

if (condition) {
  await taxios.$get('/v1/endpoint-one', config);
} else {
  await taxios.$get('/v1/endpoint-two', config);
}

[simplesmiler]

Config is closest, but it does not include the axios field.