Add ESLint and prettier

[S3B4S]

Issue: #124

I've used the following reference to set it up: https://nuxtjs.org/guide/development-tools/#eslint-and-prettier.

Highlighted rules:

• No semicolons
• 2 spaces
• Single quotes
• Trailing comma for multi-line objects / arrays
• Prop validation required (default value for prop isn't)

To run ESLint: npm run lint.
To fix some that are doable: npm run lintfix.

I've also enabled this to be run while hot reloading. But it's not yet set up to be run before commiting (git hook).

This PR also fixes any errors and warnings that have occured by adding the ESLint.

[wo1028183705]

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