Fix Number.prototype.toExponential() rounding (closes #4169)

[xcb3d]

This Pull Request fixes/closes #4169.

It changes the following:

• Replaces Rust's format!("{n:.prec$e}") in f64_to_exponential_with_precision which uses banker's rounding (round-half-to-even) with a manual implementation that uses round-half-away-from-zero to match ECMAScript semantics
• The new implementation extracts full-precision digits via format!("{n:.100e}"), then rounds the digit string correctly before formatting the exponential output
• Fixes (25).toExponential(0) returning "2e+1" instead of "3e+1" and (12345).toExponential(3) returning "1.234e+4" instead of "1.235e+4"
• The test262 return-values.js now passes in both normal and strict mode

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,901	49,902	+1
Ignored	2,222	2,222	0
Failed	840	839	-1
Panics	0	0	0
Conformance	94.22%	94.22%	+0.00%

Fixed tests (1):

test/built-ins/Number/prototype/toExponential/return-values.js (previously Failed)

Tested main commit: a49348586d90b790936dc6ab651da6aff3f60e67
Tested PR commit: bdc6c35b78e4ba2ae58188d373ce1e36daaf5f2c
Compare commits: a493485...bdc6c35

[codecov]

Codecov Report

❌ Patch coverage is 0% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.38%. Comparing base (6ddc2b4) to head (bdc6c35).
⚠️ Report is 806 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/number/mod.rs	0.00%	35 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4998       +/-   ##
===========================================
+ Coverage   47.24%   58.38%   +11.14%     
===========================================
  Files         476      559       +83     
  Lines       46892    61435    +14543     
===========================================
+ Hits        22154    35871    +13717     
- Misses      24738    25564      +826     

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[jedel1043]

The issue specifically states avoiding hacks using format!. We have a WIP implementation for this on ryu-js: boa-dev/ryu-js#50