Grammar could support the walrus operator

[alexr00]

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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) 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
• MagicPython version: commit 0b09c1fca238d22e15ac5712d03f9bf6da626f9c
• A sreenshot:
• 5-10 lines of surrounding code:

if m := x