fix: update styles and resolve vitepress conflicts

[brc-dd]

fixes #69
fixes #72

[skirtles-code]

This PR introduces some changes to the styling of the code itself:

In particular, notice the italics.

The code on the left is using the existing theme, whereas the code on the right is using this PR. The code style on the left is what you'll currently see in the Vue 3 docs (even after the VitePress 1 upgrade), but the code on the right is more consistent with the default VitePress 1 theme.

Whether this is actually a problem I'll leave for others to decide.

[brc-dd]

@skirtles-code Seems like some issue with shiki's latest version. I've created an issue on their repo: shikijs/shiki#343

[brc-dd]

There are some additional changes:

1. VitePress recently refactored markdown related stuff to a separate repo. It appears that we need to properly bundle its types. There are certain changes in the header types since the refactor, I've changed the code accordingly.
   Also, temporarily I've added @mdit-vue/types package to resolve types properly (we can remove that once vitepress#1254 is resolved).

2. VitePress has now in-built dark mode support. So there is no need to inject any custom script. I have updated the light/dark mode key accordingly.