feat: Implement Locator.nth()

[xeger]

Description

This adds more parity with Playwright locators as discussed in #7008.

Compared to elements()[n], the nth modifier promotes easier-to-debug tests by throwing an obvious error message instead of letting a matcher fail on undefined or allowing the caller to try and call some additional locator method.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

[netlify]

✅ Deploy Preview for vitest-dev ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	5eb90d8
🔍 Latest deploy log	https://app.netlify.com/sites/vitest-dev/deploys/6770426295919c000864e2f7
😎 Deploy Preview	https://deploy-preview-7137--vitest-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[xeger]

The exception message is 100% accurate now that I am using the nth engine instead of reinventing ivya/playwright 😬 .

I feel a bit bad because I am writing selector notation by hand, using string interpolation, whereas the other methods are importing from ivya locatorUtils e.g. getByRoleSelector -- so, other methods don't pollute vitest/browser with selector notation. However, ivya provides no helper method for nth engine, nor does playwright main at this time, so it's probably fine for now.

I also added first / last which are sugar, but useful. I think I'm happy with this PR. There are other useful locator methods e.g. and / or but I want to gain more experience actually using vitest + playwright to write tests before I blindly add features. If you know of any high-value or popular missing features, LMK and I will work on them in a separate PR.