Add plugin directory assets for Speculation Rules

[westonruter]

See #969

I compressed the Figma exports with Squoosh and OMGSVG.

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[domenic]

I might be missing something, but these assets seem to say "SP"; should they say "SR" instead?

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[westonruter]

@domenic the convention for the plugins has been to use the first two letters and not the initials, though I agree it's not really clear. There has been some initial discussions about changing that convention. cc @eclarke1