How to use Icons in `package.json` for Language Icon

Describe the bug
In package.json one can use icons like

...
            "commands": [
            {
                "category": "Testplan Manager",
                "command": "testplanManager.document.goTo",
                "title": "Go to",
                "icon": "$(arrow-right)"
            },
...

But that doesn't work for languages:

...
    "contributes": {
        "languages": [
            {
                "id": "testplan",
                "aliases": [ "Testplan" ],
                "extensions": [ ".tp" ],
                "configuration": "./language-configuration.json",
                "icon": {
                    "dark": "$(arrow-right)",
                    "light": "$(arrow-right)"
                }
            }
        ],
...

Because there it seems we need to use SVG files. As you provide the SVG Sprite option:

<svg>
    <use xlink:href="codicon.svg#add" />
</svg>

How to use this approach or another in package.json for language icons ?

To Reproduce
Steps to reproduce the behavior:
See description

Expected behavior
See description

Screenshots
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Environment

• OS: Mac
• VS Code Version:
  Version: 1.97.2 (Universal)
  Commit: e54c774e0add60467559eb0d1e229c6452cf8447
  Date: 2025-02-12T23:20:35.343Z (2 wks ago)
  Electron: 32.2.7
  ElectronBuildId: 10982180
  Chromium: 128.0.6613.186
  Node.js: 20.18.1
  V8: 12.8.374.38-electron.0
  OS: Darwin arm64 23.6.0

Additional context
Add any other context about the problem here.

Start with the language contribution in package.json and compare its icon fields with the command icon example. Review the codicon.svg#add sprite usage shown in the report, then establish whether $(...) syntax is supported for language icons and document the supported approach or reproduce the missing behavior.