[JENKINS-74012] Extract JavaScript block from SourceCodeViewModel/index.jelly

[yaroslavafenkin]

https://issues.jenkins.io/browse/JENKINS-74012

Testing done

Configured a pipeline that intentionally has CheckStyle violations and built it. Went to the CheckStyle warnings report, clicked on a violation that's on the lowest available line, so we can also see the scrolling behavior.

Before the change
After the change

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.71%. Comparing base (d183c39) to head (6320423).
Report is 51 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #181      +/-   ##
============================================
+ Coverage     75.39%   75.71%   +0.31%     
- Complexity       93       95       +2     
============================================
  Files            13       13              
  Lines           378      387       +9     
  Branches         30       31       +1     
============================================
+ Hits            285      293       +8     
- Misses           90       91       +1     
  Partials          3        3              

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