Upgrade to Node 20 (#1130)

Drop webusb dep that's only used for some test we largely
didn't complete. Replace the working test with a mock as its
use of the webusb dep wasn't meaningful.

Bump to working source-map-explorer (has dep fixes for Node
18+).

Author: microbit-matt-hillsdon

.github/workflows/build.yml

@@ -30,7 +30,7 @@ jobs:
       - name: Configure node
         uses: actions/setup-node@v3
         with:
-          node-version: 16.x
+          node-version: 20.x
           cache: "npm"
           registry-url: "https://npm.pkg.github.com"
       - run: npm ci