Add support for Nerd Fonts

[grynspan]

We should look into supporting Nerd Fonts as symbol substitutes when using swift test.

There's no standard way to detect what font the current terminal is using (if any). Some platform-specific mechanisms:

• For Gnome Terminal, we may be able to use GSettings.
• On Windows, GetCurrentConsoleFont() may or may not work depending on whether the pipe to SwiftPM interferes with it.
• We could read the font out of "com.apple.terminal" for Terminal.app?

Or we could just fall back to an environment variable like "SWT_NERD_FONTS_ENABLED" like we do with SF Symbols.