Unit test example for WorkItemFormGroup

Hi,

main purpose of this PR:

Include a minimal viable unit test example for React/TS components to the WorkItemFormGroup

[X] npm audit fix all security vulnerabilities
[X] bumping TS to v3.9
[X] bumping Webpack to v7
[X] bumping Node.JS in pipeline to v10.19 (LTS)
[X] adding minimal mocks for azure-devops-extension-sdk
[X] adding mocks for sample extension features
[X] adding unit tests for WorkItemFormGroupComponent to achieve 100% code coverage
[X] renaming WorkItemFormGroup manifest and html file, otherwise React testing lib will fail to import the component
[X] including unit test run into compile scripts
[X] adding jest/ts-jest configuration to package.json
[X] validated that sample extension package can be deployed to Visual Studio Marketplace, shared and installed and working
[X] validated that build pipeline successfully completes
[X] updated README.md to reflect that WorkItemFormGroup sample includes unit tests

Additional Azure DevOps Extension unit test samples can be found here: https://github.com/h2floh/azure-dev-ops-react-ui-unit-testing

microsoft / azure-devops-extension-sample