Breaking: use new language features (#94)

Namely private properties, optional chaining and the nullish coalescing
operator. This is a syntactic refactoring that makes no difference for
most consumers. The syntax is supported by all our target environments,
but if browserify is used (without babel or similar) it'll fail to
parse.

Category: change

.airtap.yml

@@ -12,3 +12,8 @@ presets:
       - airtap-electron
     browsers:
       - name: electron
+
+# Until airtap switches to rollup
+browserify:
+  - transform: babelify
+    presets: ["@babel/preset-env"]

.github/dependabot.yml

@@ -7,10 +7,12 @@ updates:
     ignore:
       - dependency-name: standard
       - dependency-name: ts-standard
-      - dependency-name: '@types/node'
+      - dependency-name: "@types/node"
       - dependency-name: voxpelli/tsconfig
       - dependency-name: typescript
       - dependency-name: hallmark
+      - dependency-name: "@babel/preset-env"
+      - dependency-name: babelify
 
       # Stay on the 3rd or 4th oldest stable release, per
       # https://www.electronjs.org/docs/latest/tutorial/electron-timelines#version-support-policy