CORP header missing in instructions how to test web extensions

[VitGottwald]

Test your web extension in vscode.dev needs one extra step to properly serve web extensions from localhost. The Cross-Origin-Resource-Policy: cross-origin header is required. This can be added by creating a serve.json:

cat > serve.json << EOF
{
  "headers": [
    {
      "source": "**/*",
      "headers": [
        {
          "key": "Cross-Origin-Resource-Policy",
          "value": "cross-origin"
        }
      ]
    }
  ]
}
EOF

Without the CORP header the extension icon and other resources do not load.

For example our HLASM extension which runs in web workers and requires SharedArrayBuffer to function does not start at all without this extra header.

[ntrogh]

@joyceerhl Can you take a look at this?