Removed jsdom dependency and test-helper.ts

[gfontorbe]

Solves #390

Removed dependency to jsdom due to licensing issues.

The dependency was only used in test-helper.ts which is not used anymore, so this file also got deleted.

[dhuebner]

@gfontorbe
It seems like test-helper.ts is still referenced in tests. I'm getting this error when running tests:

Error: Cannot find module '../utils/test-helper'
Require stack:
- /workspace/sprotty/packages/sprotty/src/lib/virtualize.spec.ts
- /workspace/sprotty/node_modules/mocha/lib/nodejs/esm-utils.js
- /workspace/sprotty/node_modules/mocha/lib/mocha.js
- /workspace/sprotty/node_modules/mocha/lib/cli/one-and-dones.js
- /workspace/sprotty/node_modules/mocha/lib/cli/options.js
- /workspace/sprotty/node_modules/mocha/bin/mocha.js

Also when searching for test-helper I found some references in configuration files.

[dhuebner]

Some test-helper references still exists in package.json and tsconfig.json. Tests are failing.