browser: add locator.isInViewport() method

Feature Description

k6/browser has no way to detect whether an element intersects the visible viewport. A CTA button can be attached, isVisible() === true, and clickable from k6's perspective, yet rendered below the fold so real users never see it — a silent failure on mobile/responsive sites.

Playwright exposes locator.isInViewport(ratio?) for exactly this case. We don't have an equivalent.

Computing the answer in userland from Locator.BoundingBox() + Page.ViewportSize() is tempting but wrong — it misses scrollable containers, CSS transforms, clip-path, iframes, and DPR/zoom. These are the cases that pushed Playwright off geometry math and onto IntersectionObserver.

Suggested Solution

locator.isInViewport(options?: { ratio?: number, timeout?: number }): Promise<boolean>

Implementation would mirror Playwright's approach — inject IntersectionObserver via Runtime.callFunctionOn — using the existing evalWithScript pattern already used by checkElementState, dispatchEvent, selectOption, etc.

Already existing or connected issues / PRs (optional)

• Playwright API: https://playwright.dev/docs/api/class-locator#locator-is-in-viewport
• Playwright upstream viewportRatio implementation: https://github.com/microsoft/playwright/blob/main/packages/injected/src/injectedScript.ts
• Companion: grafana/k6-jslib-testing#97 for the matching toBeInViewport() assertion (depends on this).