Align Intl.DateTimeFormat constructor with spec-mandated initialization order

[Vellumic]

This Pull Request fixes/closes #5335.

It changes the following:

• Swap the prototype lookup and DateTimeFormat creation to match the spec steps order
• Add a regression test for this specific case
• Add a step 1 from spec to create_date_time_format as well as comment about why it's missing

Validation

• Run (all passed)

cargo test --package boa_engine --lib --features intl_bundled -- builtins::intl::date_time_format::tests --nocapture

NOTE: This should be merged after #5324.

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,125	53,125	0
Passed	51,049	51,049	0
Ignored	1,482	1,482	0
Failed	594	594	0
Panics	0	0	0
Conformance	96.09%	96.09%	0.00%

Tested main commit: da570fd74bb6cd5b0fc01363451eb710b7ab5a0c
Tested PR commit: a3edd2fac13ec042f07237fcdac93e493bdde39d
Compare commits: da570fd...a3edd2f

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.76%. Comparing base (6ddc2b4) to head (a3edd2f).
⚠️ Report is 954 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5336       +/-   ##
===========================================
+ Coverage   47.24%   59.76%   +12.52%     
===========================================
  Files         476      590      +114     
  Lines       46892    63683    +16791     
===========================================
+ Hits        22154    38062    +15908     
- Misses      24738    25621      +883     

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