Prioritize headings inside main element

[daun]

Description

• Add the ability to try multiple heading selectors in order
• This was required in a recent project without control over markup to properly handle some weird markup
• I figured this would also be a good defensive default: first main h1, then h1
• No breaking change: it falls back to the previous version; strings are still accepted as a user option

Checks

• The PR is submitted to the main branch
• The code was linted before pushing (npm run lint)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required