Improve support for older Safari versions (except speech) (#109)

- Update build workflow to use Node 20.x
- Use webkitAudioContext is AudioContext is not defined
- Use audioCtx.createBuffer in place of new AudioBuffer
- Use createBufferSource in place of new AudioBufferSourceNode
- Fix layout that relied on aspect-ratio, closes #94 

An issue with the sample rate still remains for modspeech.

Changes made to accommodate Safari 13 and 14.

Author: microbit-robert

.github/workflows/build.yml

@@ -30,7 +30,7 @@ jobs:
       - name: Configure node
         uses: actions/setup-node@v3
         with:
-          node-version: 14.x
+          node-version: 20.x
           cache: "npm"
           registry-url: "https://npm.pkg.github.com"
       - run: npm ci