Update dependency markdown-it-anchor to v8

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
markdown-it-anchor	7.1.0 -> 8.0.0

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Release Notes

valeriangalliat/markdown-it-anchor

### [`v8.0.0`](https://togithub.com/valeriangalliat/markdown-it-anchor/blob/master/CHANGELOG.md#​800---2021-06-14) [Compare Source](https://togithub.com/valeriangalliat/markdown-it-anchor/compare/v7.1.0...v8.0.0) - Set `tabindex="-1"` on headers (\[[#​85](https://togithub.com/valeriangalliat/markdown-it-anchor/issues/85)], \[[#​86](https://togithub.com/valeriangalliat/markdown-it-anchor/issues/86)]) - Include and document accessible permalink options (\[[#​82](https://togithub.com/valeriangalliat/markdown-it-anchor/issues/82)], \[[#​89](https://togithub.com/valeriangalliat/markdown-it-anchor/issues/89)])

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