Grammar could support the walrus operator

Ninguém assumiu esta issue ainda.

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
```

Nenhum arquivo-fonte ou teste é nomeado na issue. Comece localizando a gramática do Python e as regras de escopo de tokens responsáveis por `:=` e, em seguida, compare os escopos produzidos para `if m := x` com o único token de walrus-operator pretendido; estará concluído quando o operador for destacado de forma consistente e seus sinais de dois-pontos e igual não forem mais tokens separados.

Escrita pelo modelo de indexação a partir do texto da issue.