fix: avoid ambient type leaks in declarations

[michalius]

Fixes #658.

The generated declarations currently leak two ambient types from Tinybench's build environment:

• BenchEvent extends globalThis.Event
• performanceNow is inferred as returning DOMHighResTimeStamp

These declarations fail for consumers that intentionally don't include lib.dom, including the Workerd reproduction from #658 and strict Node-only consumers.

This change:

• uses Event directly for BenchEvent
• explicitly types performanceNow as the existing NowFn (() => number)

The runtime behavior is unchanged; the changes only prevent environment-specific ambient types from leaking into the published declarations.

[michalius]

I noticed #659 was opened shortly before this PR and addresses the same issue more comprehensively, including declaration regression coverage. Closing this one in favor of #659.

Thanks. Glad to have independently confirmed the NowFn fix and the Node/Vitest 5 reproduction in #658.