Theme JS linting

[rob-howells]

Note - this is a follow up to https://github.com/accessdigital/wilson/pull/15, and so the target branch has been temporarily set to that PR. Once pull #15 is merged, the target will be updated.

Background / Motivation

• Currently there isn't any linting of the theme JS, presenting an opportunity to improve code standardisation

Approach

• Added new gulp package (gulp-eslint-new) and task to lint JS files
• Copied simplified version version of estlintrc.json from Drupal Core to use same set of lint rules, but removed any overly opinionated formatting plugins (i.e. Prettier) and unnecessary globals (e.g. jQuery) from the configuration
• Updated theme JS to adhere to new linting standards

Additional work

• Removed unused variables from gulpfile and converted functions to use ES6
• Moved button styles into Tailwind's components layer to avoid having to include button scss in the cookie scss

[rob-howells]

Is ESLint the preferred way of linting stuff? I might have to lift some of this for Wave, cos we don't really have anything solid in place right now, and it's annoying me.

But this looks good, can't beat a linting update 👍

@iamsophiesk tends to be the go to in my experience but may be implemented slightly depending on the set up of Wave. Feel free to give me a shout when you get to implementing it if I can help with any pointers