Simplify Environment Setup with a package.json file

[stephenlprice]

Environment information:

• TabPy release: 3.9

Describe the issue
The environment setup docs in CONTRIBUTING.md ask that you run the command npm install markdown-toc markdownlint to install markdown utilities. This can me simplified by adding a package.json which will help with the management of these dependencies and the new install command would be simplified to npm install.