Add Sprig Core icons

[khalwat]

Four icon variations for you to choose from, all playing on the concept of "Sprig Core" as represented by a seed that has the curly tip of the sprig to tie it together with the original Sprig icon

[bencroker]

Thanks for the icons and the much more performant implementation of HTML parsing!!

I ended up implementing this with slightly different code but the same idea. For a very large component, the parse time has gone from 3s to 0.007s (without requiring the s-verbatim tag). For an average component, the parse time has not changed and remains at 0.007s. But the fact that the parse time is not affected by the data size is ideal. I'm also happy to be able to remove the dependency on HTML5DOMDocument.