Grammar could support the walrus operator

还没有人认领这个 Issue。

From @BornForFever in https://github.com/microsoft/vscode/issues/94999

The walrus operator `:=`(an operator to allow you to make assignments in an expression, see [PEP 572](https://www.python.org/dev/peps/pep-0572)) is highlighted as 2 different colors. And I tried inspecting tokens and scopes, and found that `:` and `=` were treated as 2 tokens.

* **Editor name and version**: VS Code 1.44
* **Platform**: any
* **Color scheme**: Well illustrated with [Atom One Darker](https://github.com/christopherafbjur/vscode-theme-onedarker)
* **MagicPython version**: commit 0b09c1fca238d22e15ac5712d03f9bf6da626f9c
* **A sreenshot**: 
* **5-10 lines of surrounding code**:
```python
if m := x
```

issue 中没有指定源文件或测试。首先定位负责 `:=` 的 Python 语法和 token 作用域规则，然后将 `if m := x` 生成的作用域与预期的单个 walrus-operator token 进行比较；当该运算符得到一致的高亮，并且其中的冒号和等号不再是单独的 token 时，即表示完成。

由索引模型根据 Issue 内容生成。