Fix duplicate links not being preloaded

[daun]

Description

• Index visible links by href first and by element second
• Fix issue #104 where duplicate links would prevent preloading the link at all
• Tested in playground

Drive-by improvements

• Extract utils into separate file
• Extract observer into separate file

Checks

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

Additional information

• Closes #104

[daun]

@hirasso I'll wait for your ignore option PR before releasing this. Or feel free to tag the release yourself then.