✨ Emmet Support and Tab Behavior

#### Lack of Emmet Support:
CodeEdit does not currently support Emmet, which is a significant tool for efficient HTML and CSS coding.

#### Tab Key Behavior:
When a line of code is selected and the Tab key is pressed, CodeEdit deletes the selected line instead of indenting it. The expected behavior is that pressing the Tab key should indent the selected line, similar to the functionality in Visual Studio Code (VSC).

该 issue 未指定文件、测试或入口点。首先定位编辑器对 Tab 键的处理，以及 HTML/CSS 编辑或补全的入口点；完成的标准是 Emmet 支持可用，并且 Tab 会缩进选中的行而不是删除它们，同时针对这两种行为进行专项检查。

由索引模型根据 Issue 内容生成。