Export asyncLocalStorage to allow running it in another place

[Floriferous]

Exporting it should allow developers to write tests where you can set the context artificially.

import { asyncLocalStorage } from '@fastify/request-context';

it('should set request context', () => {
  asyncLocalStorage.run({}, async () => {
    requestContext.set('userId', 'some-fake-user-id');
    someCodeThatUsesRequestContext(); // requestContext.get('userId') will work
  })
})

Closes #211

[kibertoad]

can you add documentation with example usage (can be taken from your PR description)?

[kibertoad]

TS types need to be updated

[kibertoad]

@Floriferous do you need any help finishing this?