fix: handle empty newlines before diagrams in md

[aloisklink]

📑 Summary

Handles empty newlines before and after diagrams in Markdown.

For example, the following Markdown:

My text.

```mermaid
  my diagram
```

Should get converted to:

My text.

![diagram](./my-diagram)

However, currently, the empty newline is removed.
We can fix this by making sure that our Regex ignores newlines outside of the Mermaid diagram code.

Fixes: 9d7b4b2 (where bug was introduced)
Fixes: #413

📏 Design Decisions

Instead of using \s to find all whitespace, I'm instead using [^\S\n] to find anything that is not a non-whitespace character nor \n (basically any \s that is not \n).

In the future, it might be worth instead using https://github.com/remarkjs/remark to handle Markdown parsing, since there are probably a bunch of edge-cases that we can't handle using simplistic regex.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added unit/e2e tests (if appropriate)
• 🔖 targeted master branch