Add ESLint && adding ESLint on tests

[Rjp-Barbara]

My contribution on Chai-http was to add tests to have an impeccable code JS and to avoid the regressions. It is important to run these tests regularly for more stable code rendering.
For this, I had to use a tool named Eslint that statically analyzes the code and verifies that it follows a certain number of rules, they are obviously configurable. ESLint is available as a NPM package, which gives us the choice between a global and local installation, but here I opted for local installation, configuration at the devDependencies level and the package.json script.
In the event of a syntax error in your code, the static analysis of this code will fail, and the error in question will be sent back to you, which you could correct from the command: npm lint: fix, and the rest mannually or by adding some exception rule.
Hope this will be helpful.