fix: capture PerformanceNavigationTiming events even when window.load fires before plugin loads

[adebayor123]

Modified NavigationPlugin to capture PerformanceNavigationEvents when the plugin loads after the page does.

• Removed PerformanceObserver from eventListener.
• Added two new unit test cases and modified existing one case with respect to the new implementation change.
• Created a new app called delayed_page.html that loads the script after the page finishes loading.
• Created a new integ test case using the delayed_page app to verify that PerformanceNavigationEvents are captured.

To test the change, do the following:

• Create a temporary app (delayed_page.html) in your local repository without any other implementation changes
• Run server and either use Chrome debugger or open up local host, compare the output between index.html and delayed_page.html.
• index.html should have the navigation events, while the delayed_page.html does not.
• This indicates that with the original implementation, delayed_page properly simulates a web page loading prior to plugin
• Once this is verified, pull in the NavigationPlugin changes, and rerun the server/debugger to see that now PerformanceNavigationEvents are captured in the delayed_page app.

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[qhanam]

Does this guarantee that window.load has fired?

Or should we look for a specific event that signals the page has finished loading -- i.e., PerformanceTiming.loadEventEnd or PerformanceNavigationTiming.loadEventEnd

[adebayor123]

Discussed offline; The following line to the if statement uses PerformanceNavigationTiming.loadEventEnd

[qhanam]

Is it guaranteed that there is only one navigation timing entry?

[adebayor123]

Good catch, I will change it to iterate instead.

[adebayor123]

Actually, just found this: https://www.w3.org/TR/navigation-timing-2/
"Only the current document resource is included in the performance timeline; there is only one PerformanceNavigationTiming object in the performance timeline."

[qhanam]

Please modify the commit message to use the following conventions:

1. Use conventional commit format
   i.e.,<type>[optional scope]: <description>
2. Use the imperative mood in the subject line
   i.e., a commit message subject line should be able to complete the sentence "If applied, this commit will commit message subject line"