add option to only register "extra" extensions

[nathanwhit]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.58824% with 15 lines in your changes missing coverage. Please review.

Project coverage is 81.67%. Comparing base (0c7f83e) to head (6049b10).
Report is 308 commits behind head on main.

Files with missing lines	Patch %	Lines
core/runtime/jsruntime.rs	63.15%	14 Missing ⚠️
core/extension_set.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1145      +/-   ##
==========================================
+ Coverage   81.43%   81.67%   +0.23%     
==========================================
  Files          97      100       +3     
  Lines       23877    28996    +5119     
==========================================
+ Hits        19445    23683    +4238     
- Misses       4432     5313     +881     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.