Upgrade dependent packages to the latest version

[yhatt]

Notable changes

• Bumped Marpit to v2.1.2.
• Included some security fixes.
• github-markdown-css has bumped to v5, that is including a new style for GitHub with multi color schemes like dark mode.
  • Marp's default theme will keep inheriting styles from a pre-built GitHub light theme (github-markdown-css/github-markdown-light.css) because the design of slides should not be changed by user preferences of the color scheme. But in the next major release, we are going to change how to apply the dark color scheme through invert class: Use CSS custom properties that are provided by github-markdown-css.

Pending

• emoji-regex has shipped a new major version v10 that supports Unicode 14 emojis, but we are pending to adopt this updation because the new emojis are not supported by twemoji.