Allow runtime plugins to extend scene meta fields

[shmishtopher]

Description
Runtime plugins should be able to configure project and scene meta fields in a similar manner to custom exporters. This would enable editor plugins to easily add new gizmos as well as allowing for state-persisting hooks similar to the existing waitUntil and useDuration functions.

Proposed solution
The API should extend makePlugin (and makeEditorPlugin) with functions for configuring both project level and scene level meta fields, similar to how custom exporters provide meta fields through meta() .

import {makePlugin} from '@motion-canvas/core';

export default makePlugin({
  name: 'motion-canvas-plugin-example',
  projectMeta() {
    return new ObjectMetaField('customProjectFields', {
      propA: new BoolMetaField('propA', true),
      propB: new BoolMetaField('propB', true),
    });
  },
  sceneMeta() {
    return new ObjectMetaField('customProjectFields', {
      propC: new BoolMetaField('propC', false),
      propD: new BoolMetaField('propD', true),
    });
  },
  player(player) {
    player.onRecalculated.subscribe(() => {
      player.requestReset();
      player.togglePlayback(true);
    });
  },
});

Considered alternatives
Currently, the best way for plugins to implement stateful interaction between the editor and animation (like useDuration) is to use the vite websocket to send messages between the editor and server, and then fetch that data in the scene whenever it is recalculated.