Support packages from Typst universe

[Splines]

Resolves #9. Finally, it is possible to do something like this:

#import "@preview/physica:0.9.8": *

$ curl (grad f), tensor(T, -mu, +nu), pdv(f,x,y,[1,2]) $

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• Fetch packages from Typst Universe in the background and cache them. See Auto-download/compile packages from Typst Universe Myriad-Dreamin/typst.ts#825
• Automatically download necessary fonts and cache them.
• Add a "Preview Typst Fill" checkbox to the UI. This allows users to preview the colors used in the Typst document itself, which is very important for complex graphics with multiple colors.
• Some graphics may make use of SVG transparency. We normalize this and use opacity to be compatible with PowerPoint's SVG import.

There will certainly be some edge cases, so please open an issue should you encounter one.