d2js: add support for custom fonts

[x-delfino]

Adds support for custom fonts in d2js by passing a byte array of the ttf file

const result = await d2.compile(input, {
  fontBold: [0, 1, 0, 0], // ... contents of .ttf file
});
const svg = await d2.render(result.diagram, result.renderOptions);

example screenshot of custom font:

[alixander]

Awesome, thanks again -- just a minor comment

[alixander]

lgtm!