refactor: switch to eslint flat config

[mdjermanovic]

Switches to using eslint.config.js when linting this project.

Some notable changes:

• Upgrades to latest eslint v8 and eslint-config-eslint v9.
• Config files (eslint.config.js and rollup.config.js) and tests/util files are now also linted.
• Adds eslint-plugin-chai-friendly to replace no-unused-expressions in test files.
• eslint is now run directly instead of through Makefile.js.

Marked as "refactor" because there are few changes in the production code.

[nzakas]

LGTM.