Convert camelcase anchor links to lowercase one-word instead of kebab-case

[hirasso]

Replaces https://github.com/swup/preload-plugin/pull/115

Description

Make anchor links behave more like on GitHub. For example, with preloadVisibleLinks as input:

• before: preload-visible-links
• after: preloadvisiblelinks

Checks

• [x] The PR is submitted to the master branch
• [x] The code was linted before pushing (npm run lint)

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