Introducing CI jobs for linting and integration testing

Closes #185 (closed)

This MR introduces CI tasks that run yarn run eslint and yarn run test.

I tried to avoid any changes to production code. The only exception was formatting the src/webview_controller.js with prettier. That way the eslint job can enforce code formatting.

Successful jobs

• lint https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/jobs/569171583
• test-integration https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/jobs/569171584

Further work

• fix the eslint warnings
• create tasks for unit testing and publishing (example in gitlab-example-extension )
  • rename the unit-tests to integration_tests