fix(vitest-plugin): migrate deprecated vitest 4.1 imports

[adriencaccia]

Migrate away from deprecated vitest/runners and vitest/suite subpath imports.

Vitest 4.1 deprecated these subpath imports, printing console.warn on every load:

• vitest/runners → use BenchmarkRunner from vitest
• vitest/suite → use TestRunner static methods from vitest

Since the plugin still supports vitest 3.2 (which doesn't export from the main entry), a compat module (src/compat.ts) dynamically resolves the correct import path at runtime using top-level await:

• vitest 4.1+: imports from vitest main entry (no warnings)
• vitest 3.x: falls back to vitest/runners and vitest/suite (no warnings on 3.x)

Verified locally with codspeed run — both simulation and walltime modes work on vitest 4.1.1 (no deprecation warnings) and vitest 3.2.4 (backwards compatible).

Other changes:

• Bump vitest devDep from ^4.0.18 to ^4.1.1
• Revert vite devDep to ^7.0.0 — vite 8 + rolldown requires Node 20.19+, dev env is 20.5.1. See chore: bump Node.js minimum to 20.19+ for vite 8 dev testing #76 for the Node.js bump.
• Add jsPluginsWithTarget to rollup options for runner bundles that need esnext target (top-level await)
• Remove stale @ts-expect-error comments (vitest 4.1 fixed the getBenchFn types)
• Update instrumented test to mock ../compat instead of vitest/suite

[codspeed-hq]

Merging this PR will degrade performance by 61.43%

⚡ 9 improved benchmarks
❌ 3 regressed benchmarks
✅ 156 untouched benchmarks

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Memory	recursive fibo 15	65,624 B	19 B	×3,500
⚡	Memory	wait 500ms	64.1 KB	15 KB	×4.3
⚡	Memory	one	14.3 KB	1.6 KB	×9.2
❌	Simulation	test_recursive_fibo_10	139.5 µs	361.5 µs	-61.43%
⚡	Simulation	wait 500ms	12 ms	9.9 ms	+21.43%
⚡	Simulation	one	407.6 µs	240.5 µs	+69.46%
⚡	Simulation	fibo 15	284.2 µs	146.7 µs	+93.69%
⚡	WallTime	short body	1.9 µs	1.5 µs	+20.16%
❌	WallTime	short body	1.7 µs	1.9 µs	-12.5%
❌	WallTime	short body	1.7 µs	2 µs	-14.63%
⚡	WallTime	switch 2	348 ns	300 ns	+16%
⚡	WallTime	fibo 30	34.1 ms	29.9 ms	+14.21%

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_{Comparing feat/bump-vitest-4.1 (d104278) with feat/vite-8-support (1a48730)}