✨ Keybindings

## Overview
It should be possible to set keybinding from within CodeEdit. These keybindings should be able to reference built-in and extension commands *(See Issue #222)*.

Keybindings might be set like in [VS Code's Keyboard Shortcuts Editor](https://code.visualstudio.com/docs/getstarted/keybindings).

Extensions should be able to define keybindings similar to [VS Code Binding Rules](https://code.visualstudio.com/api/references/contribution-points#contributes.keybindings).

Users also should be able to change keybindings in the Keybindings Settings pane.

## Related Issues
- #1056
- #222

未指定源文件、测试或入口点。先阅读相关 issue #222 和 #1056 以及链接的 VS Code keybinding 参考；完成意味着 CodeEdit 可以配置用户 keybinding、将内置命令和扩展命令作为目标，并在 Keybindings Settings 面板中公开它们。

由索引模型根据 Issue 内容生成。