Add cross-platform SVG fallback support

[AprilNEA]

Problem

Currently, when SF Symbols are not available (non-macOS platforms or older macOS versions), Icon::render() returns gpui::Empty, which means icons simply don't appear.

Proposed Solution

Add a fallback mechanism that allows specifying an alternative icon source:

// Option 1: Explicit fallback method
Icon::new("star.fill")
    .fallback_svg("icons/star.svg")
    .with_size(px(24.))

// Option 2: Builder with fallback
Icon::new("star.fill")
    .or_svg("icons/star.svg")
    .with_size(px(24.))

// Option 3: Automatic mapping via trait
impl SvgFallback for MyIcon {
    fn svg_path(&self) -> &'static str {
        match self {
            Self::Star => "icons/star.svg",
            // ...
        }
    }
}

Benefits

• Cross-platform support (Linux, Windows)
• Graceful degradation on older macOS versions
• Better user experience when SF Symbols are unavailable