perf: monomorphic ResolveRequest object literals (#445 backwards-compatible)

[alexander-akait]

Re-implements the perf wins from PR #445 (constant-shape ResolveRequest
objects so V8 keeps a single hidden class on the resolver hot path)
without the breaking change for plugin authors that attach custom
properties directly to a request.

Each plugin now builds the new ResolveRequest as an explicit object
literal in a fixed property order. A new lib/util/cloneRequest.js
helper exports copyExtras(target, source) which copies any
non-canonical own string keys plus any symbol keys from the source
request, preserving the historical behavior of { ...request, ... }
for callers that pass through values via custom keys (e.g. webpack's
ResolverCachePlugin).

ParsePlugin keeps its public (source, requestOptions, target)
constructor; the partial overrides are applied via Object.assign
after the canonical literal so the { fullySpecified } use case used
by ResolverFactory does not transition the hidden class. MainFieldPlugin
keeps its alreadyTriedMainField Symbol-based recursion guard, set
after copyExtras so the current attempt's value always wins.

Adds a regression test in test/plugins.test.js that taps the resolve
hook to attach both a string-keyed and a Symbol-keyed property and
asserts both survive through the plugin chain to hooks.result.

[changeset-bot]

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Latest commit: 42b6620

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[linux-foundation-easycla]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.66%. Comparing base (42b6620) to head (9d3549d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #533      +/-   ##
==========================================
+ Coverage   96.52%   96.66%   +0.13%     
==========================================
  Files          50       51       +1     
  Lines        2651     2726      +75     
  Branches      808      813       +5     
==========================================
+ Hits         2559     2635      +76     
+ Misses         77       76       -1     
  Partials       15       15              

Flag	Coverage Δ
integration	96.66% <100.00%> (+0.13%)	⬆️

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[codspeed-hq]

Merging this PR will degrade performance by 44.79%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 58 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	concurrent-batch: Promise.all of 15 resolves (warm)	2.9 ms	5.3 ms	-44.79%

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_{Comparing claude/implement-pr445-perf-0SWZl (9d3549d) with main (3c4e574)}