feat(plugin-eslint): add support for flat config and v9

[matejchalk]

Closes #875

• The @code-pushup/eslint-plugin now supports ESLint v9 and the flat config format.
• The format is autodetected from process.env.ESLINT_USE_FLAT_CONFIG and ESLint.version.
• The config path is optional, the default configuration is discovered using a new findNearestFile utility function.
• Dropped support for inline config object, which wasn't documented or used anyway. The reason is the new format isn't serializable as JSON anymore.
• E2E tested both config formats.
• Also tested manually (installed via Verdaccio registry) in external Nx example project using ESLint 9 combined with new version of @code-pushup/eslint-config.
• This repo still uses ESLint 8 (blocked by Nx migration), which serves to verify the plugin still works with v8 via Code PushUp job in CI.

[github-actions]

Code PushUp

🤨 Code PushUp report has both improvements and regressions – compared target commit 569fa22 with source commit dee46b3.

🕵️ See full comparison in Code PushUp portal 🔍

🏷️ Categories

🏷️ Category	⭐ Previous score	⭐ Current score	🔄 Score change
Performance	🟡 51	🔴 49
Code coverage	🟡 90	🟢 90
Custom checks	🟡 67	🟡 67	–
Security	🟡 73	🟡 73	–
Updates	🟡 76	🟡 76	–
Accessibility	🟢 91	🟢 91	–
Best Practices	🟢 100	🟢 100	–
SEO	🟡 61	🟡 61	–
Bug prevention	🟢 100	🟢 100	–
Code style	🟢 100	🟢 100	–

👍 1 group improved, 👎 1 group regressed, 👍 5 audits improved, 👎 2 audits regressed, 13 audits changed without impacting score

🗃️ Groups

🔌 Plugin	🗃️ Group	⭐ Previous score	⭐ Current score	🔄 Score change
Lighthouse	Performance	🟡 51	🔴 49
Code coverage	Code coverage metrics	🟡 90	🟢 90

18 other groups are unchanged.

🛡️ Audits

🔌 Plugin	🛡️ Audit	📏 Previous value	📏 Current value	🔄 Value change
Lighthouse	Initial server response time was short	🟥 Root document took 660 ms	🟩 Root document took 430 ms
Lighthouse	Speed Index	🟥 6.3 s	🟥 7.3 s
Lighthouse	First Contentful Paint	🟥 3.1 s	🟨 2.8 s
Lighthouse	Largest Contentful Paint	🟨 3.3 s	🟨 3.6 s
Code coverage	Function coverage	🟩 92.4 %	🟩 92.7 %
Code coverage	Branch coverage	🟨 84.8 %	🟨 85 %
Code coverage	Line coverage	🟩 90.4 %	🟩 90.5 %
Lighthouse	Minimizes main-thread work	🟥 20.2 s	🟥 21.9 s
Lighthouse	Avoids enormous network payloads	🟩 Total size was 1,821 KiB	🟩 Total size was 1,820 KiB
Lighthouse	Total Blocking Time	🟥 7,560 ms	🟥 8,430 ms
Lighthouse	Time to Interactive	🟥 17.8 s	🟥 17.0 s
Lighthouse	Metrics	🟩 100%	🟩 100%
Lighthouse	Eliminate render-blocking resources	🟥 Potential savings of 1,730 ms	🟥 Potential savings of 990 ms
Lighthouse	JavaScript execution time	🟥 11.0 s	🟥 10.4 s
Lighthouse	Reduce unused CSS	🟥 Potential savings of 82 KiB	🟥 Potential savings of 57 KiB
Lighthouse	Uses efficient cache policy on static assets	🟨 28 resources found	🟨 28 resources found
Lighthouse	Server Backend Latencies	🟩 110 ms	🟩 190 ms
Lighthouse	Network Round Trip Times	🟩 10 ms	🟩 70 ms
Lighthouse	Max Potential First Input Delay	🟥 2,910 ms	🟥 2,900 ms
Lighthouse	Avoids an excessive DOM size	🟥 2,250 elements	🟥 2,247 elements

493 other audits are unchanged.

[BioPhoton]

Thx for the upgrade!