fix: Make StdClock monotonic using std::time::Instant

[abhinavs1920]

This Pull Request fixes/closes #4330 .

Previously, StdClock used SystemTime which isn't monotonic—it can jump backward when the system clock adjusts. This broke the guarantees that JsInstant is supposed to provide for engine timing.

Now StdClock uses std::time::Instant with a base instant, measuring elapsed time from that point. This makes JsInstant truly monotonic like std::time::Instant, so time never goes backward for setTimeout, setInterval, and job scheduling.

For Date objects that need actual wall-clock time, I added a separate system_time_millis() method to the Clock trait. Clean separation of concerns.

Added tests to verify the monotonic behavior works as expected.

[abhinavs1920]

Heyy @jedel1043 , I have refactored the API design as we discussed. Please take a look when ever you are available :)

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,935	49,935	0
Ignored	2,207	2,207	0
Failed	821	821	0
Panics	0	0	0
Conformance	94.28%	94.28%	0.00%

Tested main commit: 79e6549e0e82187e207883e9215535cb63ef343b
Tested PR commit: 9943af25e2721cac7df217ec1d09df7462e6f0f4
Compare commits: 79e6549...9943af2

[abhinavs1920]

Heyy I have put pushed my latest commit, but it doesn't seem to be pushed here due to some github issue. 🥲

[jedel1043]

Yep, pretty much what I expected. Thanks!

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.97%. Comparing base (6ddc2b4) to head (9943af2).
⚠️ Report is 854 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/context/time.rs	60.00%	4 Missing ⚠️
core/engine/src/builtins/temporal/now.rs	0.00%	3 Missing ⚠️
core/engine/src/builtins/date/mod.rs	66.66%	1 Missing ⚠️
...e/engine/src/builtins/intl/date_time_format/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4841       +/-   ##
===========================================
+ Coverage   47.24%   58.97%   +11.72%     
===========================================
  Files         476      563       +87     
  Lines       46892    62584    +15692     
===========================================
+ Hits        22154    36908    +14754     
- Misses      24738    25676      +938     

☔ 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.

[jedel1043]

Need to remove this dependency

[jedel1043]

Also here, needs to be removed

[jedel1043]

Thank you! Looks good