[FE]: Plugin API to communicate with other plugins

[airslice]

What

This is the development task of #374

Notice:

• Only list mounted widget/block.

What not

Implementation policy/Detail

API:

type PluginExtensionInstance = {
  id: string; // widget id, block id
  pluginId: string; // includes version
  name: string; // excludes version
  extensionId: string;
  extensionType: "widget" | "block";
};

reearth.plugins.instances: PluginExtensionInstance[]
reearth.plugins.postMessage(id: string; message: any)
reearth.on("pluginmessage", (e: { data: any, sender: string }) => {})