Added support for dark mode

[RunDevelopment]

This fixes #456.

The HTML this plugin generates is a little more complex now. We generate 3 variants of each graph now:

1. One dark theme variant.
2. One light theme variant.
3. One code block variant.

The code block is a fallback in case the mermaid library failed to load or JavaScript is disabled by the user.

CSS is used to only ever show one of the 3 variants.

Some implementation details

• I used body.light, :root[data-theme="light"] to support 0.22.x (see Is the way dark-light-theme selection is implemented part of the public API? TypeStrong/typedoc#1764) and the upcoming 0.23 (see Use data-theme attribute for themes on root element TypeStrong/typedoc#1706).
• The whole code around requestAnimationFrame is necessary because mermaid.js adds the data-processed="true" attribute before the SVG is inserted. This causes problems because we can only hide the wrong dark/light theme variant after its SVG has been inserted.

Screenshots

Dark mode:

Light mode:

No JS:

[kamiazya]

@RunDevelopment
Sorry for the late review time.

LGTM, merged.