feat: support (at least) dual themes

[Barbapapazes]

Hello 👋

This PR adds the support for dual themes in Shiki. This is very useful when a website have both a light and a dark theme.

With this feature, developers can still specifies a single theme but also two themes using an associative array.

In shiki.js, I use both variables theme and themes to align with the original Shiki API.

related to https://shiki.style/guide/dual-themes
fix #22

I'm sorry for the formatting stuff in the JS files but I could open another PR to add a JS formatter.

I'm not a PHP expert and I'm not sure how to solve this type issue.

[riasvdv]

Can you rebase on the main branch? I've updated the github actions to drop PHP 7.4 support

[riasvdv]

Thanks!

[Barbapapazes]

Thanks!

That was quick! Thanks! I will open PRs in related packages to make it available to shiki-laravel.

[Barbapapazes]

I've update laravel-markdown to my website and I could have make a mistake no mistake but I only implement one way.

With dual theme, I should not use color:#<color>;--shiki-dark:#<color> because the only way to overwrite the default theme is to use !important in the CSS. I verify the source code of Shiki website and both light and dark must be put through variables and them apply using CSS.

.dark .vp-code span {
  color: var(--shiki-dark, inherit);
}

html:not(.dark) .vp-code span {
  color: var(--shiki-light, inherit);
}

With the current implementation, the developer has to define !important to make sure the stylesheet overwrite the inline style.

We could support a new option: https://shiki.style/guide/dual-themes#without-default-color to avoid this.