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	49,687	49,687	0
Ignored	2,262	2,262	0
Failed	1,014	1,014	0
Panics	0	0	0
Conformance	93.81%	93.81%	0.00%

Tested main commit: 34ca7f1f404c2712d6897c3e39e5f5e981ce9ed6
Tested PR commit: 48fb1f8f4d14099a1af82adfc6c1af2608ce5220
Compare commits: 34ca7f1...48fb1f8

[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 57.89%. Comparing base (6ddc2b4) to head (48fb1f8).
⚠️ Report is 779 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%   57.89%   +10.65%     
===========================================
  Files         476      557       +81     
  Lines       46892    60968    +14076     
===========================================
+ Hits        22154    35300    +13146     
- Misses      24738    25668      +930     

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