feat(vitest-plugin): add vite 8 support

[adriencaccia]

Add Vite 8 to the supported peer dependency range for @codspeed/vitest-plugin.

Vite 8 replaces esbuild and Rollup with Rolldown and Oxc. This changes the internal Plugin type inheritance chain (Plugin now extends Rolldown.Plugin). The vitest-plugin only uses basic plugin hooks (name, apply, enforce, config) — none of which are affected by Vite 8's breaking changes.

Changes:

• Add ^8.0.0 to peerDependencies.vite
• Bump devDependencies.vite from ^7.0.0 to ^8.0.0
• Add rolldown as a dev dependency — needed because pnpm's strict isolation doesn't hoist rolldown (a transitive dep of Vite 8), and TypeScript needs it to resolve Rolldown.Plugin in the type chain

Vite 8 breaking changes reviewed (none affect this plugin):

• Rolldown replaces esbuild/Rollup — plugin doesn't use bundler APIs
• moduleType: 'js' required in load/transform hooks — plugin doesn't use these hooks
• Removed hooks (shouldTransformCachedModule, resolveImportMeta, etc.) — not used
• build.rollupOptions → build.rolldownOptions — not used
• esbuild → oxc config migration — not used

Note: Vite 8 requires Node.js ^20.19.0 || >=22.12.0. CI may need a Node.js bump to run integration tests against Vite 8. See #76.

Vitest v4 support was already in place (^3.2 || ^4 peer dep, ^4.0.18 dev dep) — no changes needed there.

[codspeed-hq]

Merging this PR will degrade performance by 99.97%

⚠️ 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

⚡ 3 improved benchmarks
❌ 10 regressed benchmarks
✅ 155 untouched benchmarks

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

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	wait 500ms	39.1 KB	64.1 KB	-39.05%
❌	Memory	wait 500ms	15 KB	64.1 KB	-76.63%
❌	Memory	recursive fibo 15	19 B	65,624 B	-99.97%
❌	Memory	wait 1ms	7 B	10 B	-30%
❌	Simulation	wait 500ms	8.7 ms	12 ms	-28.05%
❌	Simulation	wait 1sec	19.2 ms	22.5 ms	-14.38%
⚡	Simulation	one	585 µs	407.6 µs	+43.53%
❌	Simulation	wait 500ms	10.5 ms	11.9 ms	-12.01%
❌	WallTime	short body	1.5 µs	1.9 µs	-20.65%
❌	WallTime	switch 2	312 ns	348 ns	-10.34%
⚡	WallTime	short body	2.1 µs	1.7 µs	+21.28%
⚡	WallTime	short body	2.1 µs	1.7 µs	+25%
❌	WallTime	fibo 30	29.9 ms	34.1 ms	-12.47%

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_{Comparing feat/vite-8-support (1a48730) with main (4f97cbe)}