fix: lint import extensions and fix missing .js extensions

[matejchalk]

Now that we build using tsc for ESM runtime, imports without .js extensions have to be linted to prevent runtime errors.

[github-actions]

Code PushUp

😟 Code PushUp report has regressed – compared target commit 6abe0e9 with source commit 284f755.

🕵️ See full comparison in Code PushUp portal 🔍

🏷️ Categories

🏷️ Category	⭐ Previous score	⭐ Current score	🔄 Score change
Performance	🟡 54	🔴 41
Code coverage	🟢 91	🟢 91	–
Security	🟡 81	🟡 81	–
Updates	🟡 80	🟡 80	–
Accessibility	🟢 91	🟢 91	–
Best Practices	🟢 100	🟢 100	–
SEO	🟡 61	🟡 61	–
Bug prevention	🟢 100	🟢 100	–
Code style	🟢 100	🟢 100	–

👎 1 group regressed, 👎 4 audits regressed, 11 audits changed without impacting score

🗃️ Groups

🔌 Plugin	🗃️ Group	⭐ Previous score	⭐ Current score	🔄 Score change
Lighthouse	Performance	🟡 54	🔴 41

16 other groups are unchanged.

🛡️ Audits

🔌 Plugin	🛡️ Audit	📏 Previous value	📏 Current value	🔄 Value change
Lighthouse	Largest Contentful Paint	🟨 3.1 s	🟥 4.1 s
Lighthouse	First Contentful Paint	🟨 2.8 s	🟥 3.7 s
Lighthouse	Speed Index	🟥 6.4 s	🟥 8.5 s
Lighthouse	Time to Interactive	🟥 16.6 s	🟥 17.6 s
Lighthouse	Minimizes main-thread work	🟥 23.0 s	🟥 25.9 s
Lighthouse	JavaScript execution time	🟥 9.0 s	🟥 10.9 s
Lighthouse	Total Blocking Time	🟥 10,290 ms	🟥 11,570 ms
Lighthouse	Metrics	🟩 100%	🟩 100%
Lighthouse	Max Potential First Input Delay	🟥 3,410 ms	🟥 3,770 ms
Lighthouse	Avoids enormous network payloads	🟩 Total size was 1,818 KiB	🟩 Total size was 1,819 KiB
Lighthouse	Initial server response time was short	🟩 Root document took 400 ms	🟩 Root document took 580 ms
Lighthouse	Eliminate render-blocking resources	🟥 Potential savings of 1,020 ms	🟥 Potential savings of 1,170 ms
Lighthouse	Server Backend Latencies	🟩 160 ms	🟩 120 ms
Lighthouse	Uses efficient cache policy on static assets	🟨 28 resources found	🟨 28 resources found
Lighthouse	Network Round Trip Times	🟩 20 ms	🟩 20 ms

570 other audits are unchanged.