Add support for custom Categories

[shduke]

Summary

Changing SymbolCategory from an enum to a struct allows for easier extension. Users can now define their own SymbolCategory without having to modify the symbol list text file.

Example

let circleCategory = SymbolCategory("circles", systemImage: "circle", isMember: { $0.name.contains("circle") })
SymbolPicker(symbol: $symbol, categories: [circleCategory])

This builds on #13 and keeps its functionality

[Fluffik3666]

Please add!