feat: Add slick code styling

[matthew-carroll]

Description

Flutter and Dart developers want to show Flutter and Dart code. It's important that the code styling experience be top notch.

slick_slides already has great code styling. Port that approach to flutter_deck.

Requirements

• Add dependency on syntax_highlight
• Integrate with existing code display
• Look for places to take the existing code display to a higher level with that package

[matthew-carroll]

@mkobuolys - Do you have any thoughts or input on this before I begin?

[mkobuolys]

Hey @matthew-carroll, here are my thoughts:

1. The current FlutterDeckCodeHighlight must be revisited and its implementation, ideally, should be replaced. If that's not possible, I am completely fine removing/replacing this widget and introducing a breaking change if needed.
2. The flutter_highlight dependency must be removed.
3. The current slide theme should be applied to the code highlighting (light/dark), but it should also support theme override if needed (this is how all the theming works in flutter_deck).
4. It would be nice if the code could scale and resize based on the available space.
5. Code change animation (similar to in slick_slides) would be a nice addition.

[mkobuolys]

Implemented with #180