feat: add elementIndex step option for targeting specific elements

[DavertMik]

Summary

• Add elementIndex step option to select a specific element when multiple match a locator
• Supports positive (1-based), negative (-1 = last), and 'first'/'last' aliases
• When only one element matches, elementIndex is silently ignored
• Overrides strict: true mode when set (user explicitly chose which element)
• Implemented across Playwright, Puppeteer, and WebDriver helpers
• Add documentation page for element selection and strict mode

import step from 'codeceptjs/steps'

I.click('a', step.opts({ elementIndex: 2 }))    // 2nd element
I.click('a', step.opts({ elementIndex: -1 }))   // last element
I.click('a', step.opts({ elementIndex: 'last' })) // same as -1

Test plan

• All 7 new elementIndex tests pass for Playwright
• All 7 new elementIndex tests pass for Puppeteer
• Existing strict mode tests still pass (no regressions)
• Existing click/fillField/selectOption/checkOption tests pass
• Run WebDriver tests when Selenium available

🤖 Generated with Claude Code