Fix PowerShell highlighting of built-in constant variables

[o-l-a-v]

Related VSCode PR: microsoft/vscode#298570

PR Summary

Use a TextMate scope for PowerShell built-in constant variables that most themes actually have defined in tokenColors.

I see that C#, Ruby and Go TextMate files uses variable.other.constant.<language>, so I went with that.

• https://github.com/search?q=repo%3Amicrosoft%2Fvscode%20variable.other.constant.&type=code

PR Context

For a long time, PowerShell highlighting for built-in constant variables has been broken in VSCode.

Some related issues:

• 2024-03-16: Some variables are not highlighted ($Error, $PSVersionTable) vscode-powershell#4944
• 2024-03-17: Variables that are built-in constants are not highlighted, like $Error and $PSVersionTable #218
• 2025-04-03: The automatic variable $PSHOME is not highlighted vscode-powershell#5166

I believe I've tracked it down to the PowerShell TextMate using a TextMate scope most themes does not have a definition for:

The official Dracula theme even have a workaround just for this https://github.com/dracula/visual-studio-code/blob/main/src/dracula.yml:

  - name: Powershell constants mistakenly scoped to `support`, rather than `constant` (edge)
    scope:
    - support.constant
    settings:
      fontStyle: normal
      foreground: *PURPLE