Allow specify frame index in the "switchToIFrame" driver method

[aik099]

See https://github.com/minkphp/Mink/issues/866 (PR 3 of 4)

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If the https://github.com/minkphp/MinkSelenium2Driver/pull/382 happens to be merged before this PR, then we need to:

1. rebase this PR on top of master branch;
2. replace the if ($name) { line in the \Behat\Mink\Driver\Selenium2Driver::switchToIFrame method with the if ($name && !is_int($name)) { line.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4ca4083) 90.19% compared to head (6e8b226) 90.19%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #378 +/- ## ========================================= Coverage 90.19% 90.19% + Complexity 168 167 -1 ========================================= Files 1 1 Lines 469 469 ========================================= Hits 423 423 Misses 46 46 ```

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[aik099]

Closing as per https://github.com/minkphp/Mink/issues/866#issuecomment-1960368999 .