feat: add generic types to get story functions

[AdrianFahrbach]

Pull request type

Jira Link: No Jira Link available.

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Other (please describe):

How to test this PR

This PR introduces type changes to .get, .getAll, .getStory and .getStories. The new behaviour can be tested by using those functions with generic types like this:

import StoryblokClient from 'storyblok-js-client'

interface MyCustomType {
  foo: bar;
}

let client = new StoryblokClient({
  accessToken: '<YOUR_SPACE_ACCESS_TOKEN>',
})

const res = client.getStory<MyCustomType>('home')

If you hover over res in your IDE it should be of type ISbStory<MyCustomType>

What is the new behavior?

This PR adds optional generic types to these functions to allow for easier typing when using this client.
With ISbStory and ISbStories there already are generic interfaces for typing stories, those had to be manually assigned for now though. ISbResult got generic in this PR.