Naive support for system fonts

[dsgallups]

Objective

I was working on this and noticed a superior implementation in #16365. However, I am unsure of the general consensus on loading system fonts. This PR uses the existing Font asset to query the system directly for fonts.

• Fixes Support using system fonts #1325

Solution

• Update Font to include a Font::System variant.
• Update TextPlugin to include a load_system_fonts parameter

Testing

• In terms of testing, this can vary wildly across platforms. Would like some advice on a general approach to satisfy the runners. I'm unsure what they've got haha
• examples/system_fonts

---

Showcase

A font can be added with with the new Font::System variant:

let font: Handle<Font> = fonts.add(Font::System {
    families: vec![
        Family::Name("Liberation Sans".to_string()),
        Family::Name("Ubuntu".to_string()),
        Family::Name("Noto Sans".to_string()),
    ],
    weight: Weight::NORMAL,
    stretch: Stretch::Normal,
    style: Style::Normal,
});

Note

I would love to enable System Fonts through this PR or via #16365. Please let me know what I need to do to make this possible!

[dsgallups]

Just found #14150 as well. I'm going to address the feedback there since my implementation appears pretty similar.

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