Add other generated ligatures: ẞ, ß, ð, Þ, þ

> Note that I've generated something with ChatGPT (@Carreau's prediction 😂):
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>
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> This was using the prompt:
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> Take this image, and generate an image containing ẞ, ß, Æ, Œ, a, æ, œ, ð, Þ, þ characters in the same style. The a and æ should use double-story form.
>
> Along with the original [handwriting sample](https://github.com/ipython/xkcd-font/blob/master/xkcd-script/generator/handwriting_minimal.png).
>
> The only existing form that I could find anywhere in XKCD is https://xkcd.com/2763/ (Æ)

_Originally posted by @pelson in [#66](https://github.com/ipython/xkcd-font/issues/66#issuecomment-4311093983)_