Add tests for ESLint rules

[SaladFork]

Resolves #6. Thanks for tagging with Hacktoberfest and having good documentation!

This PR adds tests for all the rules in the project except for no-filter-instead-of-find, which already had tests. It also adds a test framework and test commands for running said tests (I picked Jest but it's easy to swap to whatever runner you'd prefer).

As part of writing these tests, I believe I found two bugs. I commented out the failing case and added a FIXME in both relevant tests:

• classbody-starts-with-newline throws an error for this code which the documentation suggests should pass:

export class SomeClass {
    /**
     * Some comment
     */
    constructor() {
        super()
    }
}

• no-duplicated-chains passes this code but the documentation suggests it should throw an error:

function render() {
    return (<div className={this.props.className}>
        <p>{this.props.text}</p>
    </div>);
}

This PR also corrects a few minor mistakes in the documentation.