[WIP] HTML Preview

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

Ensure that HTML previews are displayed when converted to HTML.

• Use mermaid.js properly to make sure mermaid is displayed properly
• prism.js should be used properly to highlight source code properly.
• Ensure that "```math" is properly converted to "" and displayed properly
• Ensure that samples/sample.md is properly previewed when converted to HTML

MarkDown から HTML に変換するとき、

ルール1.

のブロックは、

に変換

ルール2.

のブロックは、

に変換

Fixes #20.

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