fix: allow specifying an icon for config enums (fixes #123786)

[locactus]

Fixes #123786

Summary

This PR adds support for displaying icons alongside configuration enum values in the Settings UI dropdown. Extension authors can prefix markdownEnumDescriptions with a codicon reference (for example $(zap) Fast mode) and the icon will render in the custom select box dropdown.

Changes

• Add optional icon?: ThemeIcon to ISelectOptionItem.
• Render a codicon <span> in SelectListRenderer when an option includes an icon.
• Add styles for .option-icon in selectBoxCustom.css.
• Parse leading $(icon-id) markers from markdownEnumDescriptions in settingsWidgets.ts and validate them against the codicon registry.
• Wire the parsed icon through settingsTree.ts so enum settings use the custom select box dropdown when icons are present.
• Add browser and unit coverage for icon extraction and rendering.

Validation

• TypeScript typecheck passed.
• ESLint passed on the changed TypeScript files.
• Node unit tests passed.
• Format check passed with no reformatting needed.
• git diff --name-only origin/main...HEAD contains only the expected 7 changed files for this fix.
• No package.json or tsconfig*.json changes.

How to test

1. Install an extension (or add a test setting) that uses markdownEnumDescriptions with a leading codicon, for example:

   "markdownEnumDescriptions": [
     "$(zap) Fast mode",
     "$(shield) Safe mode"
   ]

2. Open Settings, find the setting, and open the dropdown.
3. Verify the codicon appears to the left of each option label.
4. Verify options without icons still render correctly.

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@rzhao271

Matched files:

• src/vs/workbench/contrib/preferences/browser/settingsTree.ts
• src/vs/workbench/contrib/preferences/browser/settingsWidgets.ts
• src/vs/workbench/contrib/preferences/test/browser/settingsWidgets.test.ts

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