feat(js-sdk): add script loader to set up Sentry Javascript SDK

[buenaflor]

📜 Description

This will be merged into feat/js-sdk-integration branch, not main

💡 Motivation and Context

PR Nr 1 of JS SDK integration

💚 How did you test it?

Unit test, manual check of html document

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• No new PII added or SDK only sends newly added PII if sendDefaultPii is enabled
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

#skip-changelog

[github-actions]

	Messages
📖	Do not forget to update Sentry-docs with your feature once the pull request gets approved.

Generated by 🚫 dangerJS against 59b4ab8

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.

Project coverage is 85.92%. Comparing base (dd16d3e) to head (59b4ab8).

Files with missing lines	Patch %	Lines
...ib/src/web/script_loader/sentry_script_loader.dart	75.00%	3 Missing ⚠️
...tter/lib/src/integrations/web_sdk_integration.dart	83.33%	1 Missing ⚠️

Additional details and impacted files

@@                     Coverage Diff                     @@
##           feat/js-sdk-integration    #2406      +/-   ##
===========================================================
+ Coverage                    85.06%   85.92%   +0.86%     
===========================================================
  Files                          257       84     -173     
  Lines                         9191     2891    -6300     
===========================================================
- Hits                          7818     2484    -5334     
+ Misses                        1373      407     -966     

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