[Monaco Editor] Fix text selection not visible in High Contrast Mode

[Copilot]

Issue

In High Contrast Mode, text selection in Monaco Editor is not visible across multiple PowerToys components:

• Peek
• Registry Preview
• File Explorer Addons

This makes the editor difficult to use for users with accessibility needs who depend on High Contrast Mode.

Solution

Added CSS styles that specifically handle text selection in High Contrast Mode to:

1. The main Monaco template (src/Monaco/index.html)
2. The Registry Preview implementation (src/modules/registrypreview/RegistryPreviewUILib/Assets/RegistryPreview/index.html)

The implementation uses:

• The @media (forced-colors: active) media query to detect when the system is in High Contrast Mode
• System colors Highlight and HighlightText to ensure proper contrast in all High Contrast themes
• The !important directive to override default Monaco styles

/* Fix for text selection in High Contrast Mode */
@media (forced-colors: active) {
    .monaco-editor .focused .selected-text {
        background-color: Highlight !important;
        color: HighlightText !important;
    }
    .monaco-editor .selected-text {
        background-color: Highlight !important;
        color: HighlightText !important;
    }
}

These changes ensure that text selection is visible regardless of the High Contrast theme applied by the user.

Fixes #13.

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