Added initial documentation and tests

[rasmusjp]

This PR contains the initial work of documenting the Public APIs and testing the library

Added the following libraries

• eslint - To ensure code consistency
• @typescript-eslint/parser - Typescript parser for ESLint
• eslint-config-standard-with-typescript - Standard rules for ESLint
• @microsoft/api-extractor - Extracts and validates the public API surface
• @microsoft/api-documenter - Generates markdown/yaml from the api-extractor files
• mocha - Test runner
• chai - Assertions
• sinon - Test spies, fakes and mocks
• axios-mock-adapter - Easily mock axios requests

I've run ESLint and fixed all errors, there's still a couple left but I've changed them to warnings for now since they are not major problems.

I've also

• Updated documentation for Content and Media delivery.
• Moved Content and Media management to classes since @microsoft/api-extractor don`t work that great for nested objects.
• Added tests for Content Management Client
• Added examples for creating and updating Content and Media through the Management Client

To test the documentation run

> npm run install
> npm run build
> npm run docs
> npx markserv docs/api

The tests can be run by invoking

> npm run install
> npm run build
> npm run test

[sitereactor]