Grammar could support the walrus operator

還沒有人認領這個 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 中沒有指定原始檔案或測試。先定位負責 `:=` 的 Python 語法和 token 範圍規則，然後將 `if m := x` 產生的範圍與預期的單一 walrus-operator token 進行比較；當該運算子獲得一致的醒目顯示，且其中的冒號和等號不再是分開的 token 時，即表示完成。

由索引模型根據 Issue 內容生成。