✨ Snippets

### Description

Users should be able to make and insert code snippets to insert into their open file at the current cursor position.

Snippets should be able to support wildcards.

Snippets should have a completion, so when a set of characters are typed within a given scope, it will suggest the snippet without having to open the Snippets overlay.

Additionally, extensions should be able to add snippets.

We might include this in a larger library feature at a later point in time.

The snippets/library overlay could be accessed by a keyboard shortcut and/or a plus icon button in the toolbar.

> [!NOTE]
> There may require additional functionality to be build into CodeEditSourceEditor to support some of this, so we may need to do some of this after this is done.

### Blockers

- CodeEditSourceEditor completion service and menu
- CodeEditSourceEditor wildcard support

### Screenshots

该 issue 将 CodeEditSourceEditor 的 completion service 和 menu，以及 wildcard 支持列为阻塞项，但没有提供实现文件或测试。首先阅读 CodeEditSourceEditor 的这些区域，并明确 snippet 库、插入、completion、wildcard 和扩展 API 的范围。完成标准是，所列出的 snippet 创建、光标插入、作用域 completion、wildcard、扩展和 overlay 行为均能正常工作。

由索引模型根据 Issue 内容生成。