chore: improve ESLint configuration

[toddeTV]

1. Limit max line length

Enforce a maximum line length limit, as currently requested visually by the VSCode settings.
This can be done using the ESLint rule: https://eslint.org/docs/latest/rules/max-len

2. Revise unused variables handling

Unused variables should be revised:

• Variables that are not used but start with the _ prefix should trigger a warning.
• Unused variables without this prefix should trigger an error.

To achieve this, we should disable the currently active unused-imports/no-unused-vars rule and enable the ts/no-unused-vars rule with settings accordingly.