Mermaid and MathJax for nvALT

Another update to nvALT. This is probably going to keep happening. Mermaid, MathJax, and updated documentation are included this time.

The nvALT project page is the documentation now. Installation, search, tagging, the editor shortcuts, themes, and the Apex syntax the preview actually renders are all on one page.

The preview also typesets math and draws Mermaid diagrams.

Math

Apex already understood the delimiters. The preview now runs MathJax on them.

Inline math sits in the sentence:

The area of a circle is $A=\pi r^2$.

A display equation gets its own lines:

$$
E = mc^2
$$

\(...\) and \[...\] work the same way. $40 stays a price, because Apex treats a dollar sign followed by a digit as currency.

Diagrams

A fenced block tagged mermaid becomes a diagram:

```mermaid
flowchart LR
  A[Note] --> B[Preview]
  B --> C[MathJax]
  B --> D[Mermaid]
```

That is the one diagram type the preview draws. A Graphviz block stays a code block.

Bundled, and only when the note needs them

MathJax 4 and Mermaid 12 ship inside the app. The preview does not call out to a CDN, so a note still renders on a plane.

A note with no math and no Mermaid fence does not load either library. Open a note that has one of them and that library comes along. Open a note that has both and you get both.

Each time the preview updates, it reloads the whole page. MathJax and Mermaid start over with that reload. The scroll position is the thing that carries across.

The HTML source view, and Save Preview HTML, stay as the Apex markup. The libraries are there for the live preview.