feat(monospace): add settings schema + safe defaults

getDefaults(), getActiveMonoFont(), isLigaturesEnabled() with TDD.

src/main/settings/monospaceSettings.js

@@ -0,0 +1,21 @@
+'use strict';
+
+const FAMILY_BY_KEY = {
+  'jetbrains-mono': 'JetBrains Mono',
+  'fira-code': 'Fira Code',
+};
+
+function getDefaults() {
+  return Object.freeze({ monospaceFont: 'jetbrains-mono', monospaceLigatures: false });
+}
+
+function getActiveMonoFont(settings) {
+  const key = settings && settings.monospaceFont;
+  return FAMILY_BY_KEY[key] || 'JetBrains Mono';
+}
+
+function isLigaturesEnabled(settings) {
+  return Boolean(settings && settings.monospaceLigatures === true);
+}
+
+module.exports = { getDefaults, getActiveMonoFont, isLigaturesEnabled, FAMILY_BY_KEY };