Feature/emulsify linting setup

[jani-koski]

https://wunder.atlassian.net/browse/PSW-1

• A linting configuration that uses the same settings as the Emulsify theme, but I've taken out all the non-relevant things, such as any configuration that is related to React.
• For unifying purposes, I changed all the config files to .rc with JSON syntax.
• On the Emulsify theme, .stylelintrc-file is located in the Webpack folder, and there possibly is a practical reason for that. But at this point, as we don't even have Webpack on the repo, I just added the stylelint file to the root of the project.

[dbruvers]

Two comments here:

1. The config file name should be .estlintrc.json. From Eslint docs:

ESLint supports configuration files in several formats:

    JavaScript - use .eslintrc.js and export an object containing your configuration.
    ...
    JSON - use .eslintrc.json to define the configuration structure. ESLint's JSON files also allow JavaScript-style comments.
    Deprecated - use .eslintrc, which can be either JSON or YAML.

2. When we run npm run lint from the project root then ESlint finds ./web/.eslintrc.json. We need to discuss whether it makes sense to have an additional .eslintrc.json file at the project root.

[jani-koski]

I would say that things generally are looking pretty good, or at least something that we can start off with. I did a few more changes though:

• Fixed style linting paths (lint:css) paths in package.json
• Installed stylelint-prettier package. I noticed that we had not yet taken into account running prettier rules for css files. We had only disabled formating related rules, but not installed or configured a package which runs prettier as a stylelint rule; stylelint-prettier does exactly that.

Now, at this point someone should probably test that the linting setup works as desired:

• Eslint runs in a way that formatting related rules are handled by prettier
• Stylelint runs in away that formatting related rules are handled by prettier

[joshua-scott]

Works well for me, I think this is a great starting point. Made one small suggestion.

[joshua-scott]

Instead of adding expect/it/describe as globals, we could instead add jest to env: "env": { "jest": true }