feat: add support for paused slides using `<!-- pause -->`

[xukai92]

Changes Introduced

• within each page, one now can use <!-- pause --> to pause
• each page is broken down to contain the content so far until each pause

[maaslalani]

Oh this is super interesting, I almost wonder if we should use (+++):

+++

as the delimiter.

This is a very clever and elegant solution to the reveal problem! Thank you so much!

[xukai92]

I was mostly drawing inspiration from https://mfontanini.github.io/presenterm/guides/basics.html#pauses and I prefer it over +++ or *** because this only affects how the Markdown files is presented as slides by slides and the content stays unchanged when it is viewed by other Markdown viewers.