Skip link placement: All page content should be contained by landmarks

### Product

axe-core

### Product Version

4.8.2

### Latest Version

- [X] I have tested the issue with the latest version of the product

### Issue Description

#### Expectation

A skip link placed outside a landmark should not trigger a violation based on [region](https://dequeuniversity.com/rules/axe/4.0/region) rule, i.e. All page content should be contained by landmarks

> It is best practice to contain all content excepting skip links, within distinct regions such as the header, nav, main, and footer.

#### Actual

If a skip link is not the first href defined on the page, a region violation is triggered due to the [isSkipLink check](https://github.com/dequelabs/axe-core/blob/853a8ead80757f2da51ce61effc1f31c3da653a4/lib/commons/dom/is-skip-link.js#L26).

#### How to Reproduce

1. open up [https://www.tax.service.gov.uk/help/terms-and-conditions](https://www.tax.service.gov.uk/help/terms-and-conditions)
2. you should see a cookie consent banner
3. run axe with the banner open
4. the skip link on the page won't be detected as a skip link and you will get a violation
5. close the banner (i.e. click reject additional cookies, click hide cookies message)
6. rerun axe
7. the skip link is correctly detected and you will not get a violation

#### Additional context

The skip link is placed after the cookie banner as per [GOV.UK design system guidance on Cookie Banner placement](https://design-system.service.gov.uk/components/cookie-banner/#how-it-works)

> Position the cookie banner after the opening tag and before the ’skip to main content‘ link.