Refactoring: API for defining new frame sources

[bkeepers]

This pull request aims to create a public API for defining new frame sources, and refactors exising layer sources to use it.

• Create an API for defining new frame sources

import { defineFrameSource, type BaseLayer } from 'editly';

type GifLayer extends BaseLayer {
  type: 'gif';
  path: string;
}

export default defineFrameSource<GifLayer>('gif', async ({ params, width, height }) => {
  const { path } = params;

  // Initial setup here, like loading the file

  return {
    async readNextFrame(progress, canvas) { /* Draw on `canvas` or return RGBA data */ },
    close() { /* Do any cleanup here */ }
  };
});

Later:

• Expose a public API to register new frame sources
• Refactor handleLayer in config.ts to move validations into individual layer types
• Create an example
• Update README
• A plugin API so we can extract existing frame sources into separate packages with their own dependencies