Add eslint rules

[jochenklar]

This PR adds rules to .eslintrc.js to ensure a consistent JS style. The rules cause a lot of issues which need to be resolved after we agreed on the rules and probably after other branches have been merged.

[CalamityC]

These three ESLint rules can clash in certain cases:

'multiline-ternary': ['error', 'never'],
'max-len': ['error', { code: 120 }],
'react/jsx-curly-newline': ['error', { multiline: 'require' }]

• multiline-ternary: never → ternaries must stay on one line
• max-len: 120 → line length is limited
• react/jsx-curly-newline → multiline JSX expressions should be split across lines

For longer ternaries (especially in JSX), this can lead to situations where:

• breaking the ternary is not allowed
• keeping it on one line exceeds max length

Just something to be aware of when formatting conditional expressions.

[CalamityC]

These rules can interact in a slightly confusing way:

'multiline-ternary': ['error', 'never'],
'indent': ['error', 2, { offsetTernaryExpressions: true }]

multiline-ternary: never → ternaries must stay on one line
offsetTernaryExpressions: true → controls indentation for multiline ternaries

Since multiline ternaries are not allowed, the offsetTernaryExpressions option never really comes into effect.

Just something to keep in mind when looking at the config.

[CalamityC]

After reading through PR#1565, especially this part of STYLE.md,
I'm pretty convinced that we don't want the rule
'multiline-ternary': ['error', 'never'],

[jochenklar]

@CalamityC https://eslint.org/docs/latest/rules/multiline-ternary

We will remove { offsetTernaryExpressions: true }.

[jochenklar]

@CalamityC I added some rules for the imports. It is now:

import React from 'react'
import PropTypes from 'prop-types'
// all other external imports

// imports from rdmo

// components from rdmo

// imports from ..

// components from ..

// imports from .

// components from .

The trick is that components have upper case filenames. I know this is not the case for index.js, but I can live with that.

[jochenklar]

Closed in favor of #1572.