fix: add getLoader method to SpineLoader, export atlas and spine loaders

[f0nar]

Solve issue #522

Add ability to use pixi-spine with pixi.js-legacy by loaders exporting, so you can register them in correct context

import * as PIXI from "pixi.js-legacy";
import { SpineLoader } from '@pixi-spine/loader-uni';
import { spineTextureAtlasLoader } from '@pixi-spine/loader-base';

PIXI.extensions.add(spineTextureAtlasLoader);
PIXI.extensions.add(new SpineLoader().getLoader());