Feat: Add fill-in-blank and copy feature to the codesplit block

[jasongao97]

This PR introduces a new Codesplit component with the following features:

• allows for toggling visibility of answers which were defined as <span class="blank">{{code}}</span>
• a 'copy' button to copy the whole block in raw text to clipboard

Screen.Recording.2024-09-13.at.00.01.47.mov

Note: Code snippet without any blank will only have the 'copy' button

Some other changes:

• code languages are now displayed
• the build process now handles HTML content within code blocks more accurately

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[shiffman]

Thank you for this @jasongao97, it's really terrific! I have two small comments:

1. Since the entire book is JavaScript I don't think it's necessary to show the code language. It's visually very prominent and I think distracting. I'm open to discussing this though! If it's important to include maybe we can change the design to be more subtle. @tuantinghuang curious for your thoughts.

2. This is maybe a silly thing but would it be possible for the "copy" button to copy the source code with the blanks if "reveal answer" is not selected?