refactor: events/commands/types restructured

[Charlie-XIAO]

Events

All events are written in core/events.ts. Each event is separated by a single-dashed comment block, marking possible pairs of emitters/listeners. Each event has a payload, unless void. By default the payload types are not exported, except in special cases. Each event is an object that can contain the one or more of the following properties: on which corresponds to the listener, toCanvas which means emit to canvas, toManager which means emit to manager. In the future if needed, there might toAll. To use events, import { events } from "/path/to/core"; then do e.g. events.renderWidgets.on(...), events.updateSettings.toCanvas(...), etc.

Commands

All commands are written in core/commands.ts. Payload types are inlined. To use commands, import { commands } from "/path/to/core";, then do e.g. commands.setRenderReady(), etc.

Types (Shared)

All backend types are written in files under types/backend/, corresponding to the module that the type belongs to in the backend. For instance, types that comes from the config module are under types/backend/config.ts. Types related to events are commands are exceptions - they are colocated with the events and the commands. For frontend types, currently they should be declared under canvas and manager separately. The only item that is under types/frontend.ts for now will be removed in a subsequent PR to under manager. All types are re-exported under index.ts, which may also provide additional utilities as needed. To use shared types, import { ... } from "/path/to/types";. Non-shared types that are respectively under canvas and manager are not touched. Types for events and commands should be used via events.xxx and commands.xxx, where events and commands should be imported in the ways specified above.