feat(monospace): ExportCss embeds woff2 as base64 in CSS

Self-contained CSS for HTML export and print-preview iframe.

src/main/ExportCss.js

@@ -0,0 +1,34 @@
+'use strict';
+
+const fs = require('fs');
+
+function toDataUri(filePath) {
+  const buf = fs.readFileSync(filePath);
+  return `data:font/woff2;base64,${buf.toString('base64')}`;
+}
+
+function build({ activeFontPath, family, weight = 400, ligatures = false }) {
+  const features = ligatures ? 'normal' : "'liga' 0, 'calt' 0, 'dlig' 0";
+  const faceBlock = activeFontPath
+    ? `@font-face {
+  font-family: '${family}';
+  font-weight: ${weight};
+  font-style: normal;
+  font-display: swap;
+  src: url('${toDataUri(activeFontPath)}') format('woff2');
+}
+`
+    : '';
+
+  return `${faceBlock}code, pre, kbd, samp {
+  font-family: '${family}', monospace;
+  font-feature-settings: ${features};
+}
+pre, code {
+  white-space: pre;
+  tab-size: 4;
+}
+`;
+}
+
+module.exports = { build };