Implement eslint and lint codebase

[DCSBL]

Continuing on #83

This PR:

• Implements eslint via the default config of https://github.com/athombv/eslint-config-athom
• Removes prettier (as this has conflicting rules, and was not used in CI)
• Adds a pre-commit-config file; Install pre-commit, and run pre-commit install. This runs some quick checks before each commit to prevent 'invalid' commits. Can be ignored via --no-check flag per commit or even not installed at all.

All non auto-fixable rules are disabled, and can be enabled step by step.

Waits for #110