Add ESLint to CI

[dduugg]

Summary

Adds yarn lint as a required PR check and replaces the broken eslint-config-gusto setup with a minimal ESLint flat config appropriate for this Node.js plugin.

Why the old config was broken:

• .eslintrc.js used module.exports which fails in an ESM project ("type": "module")
• eslint-config-gusto is a React/TypeScript config — inappropriate for a Node.js plugin, and was the source of the axe-core license violation and typosquat security alerts on the previous PR

Changes

• Replace eslint-config-gusto with eslint + @eslint/js (ESLint's own recommended config)
• Replace .eslintrc.js with eslint.config.js (ESLint 9 flat config, ESM-compatible)
• Delete .prettierrc.js (was re-exporting Prettier config from eslint-config-gusto)
• Fix two lint errors surfaced by the new config in verifyConditions.js: useless null initializer and unused catch variable
• Split lint and jest into separate parallel jobs so failures in one don't mask the other