Grammar could support the walrus operator

[alexr00]

From @BornForFever in microsoft/vscode#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 0b09c1f
• A sreenshot:
  
• 5-10 lines of surrounding code:

if m := x