Add warning for section not containing heading element (W0012)

[shamahdev]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

This feature adds a warning to a <section> not containing heading element which it should be contained because a <section> doesn't has other specific semantic element to represent it. Resolves: #89

Link(s)

• https://developer.mozilla.org/en-US/docs/Web/HTML/Element/section

Screenshot(s)

Checklist:

• I have thoroughly read the CONTRIBUTING guidelines.
• I understand my pull request will be thoroughly reviewed at high detail.
• I understand the work in my pull request will only be available in the next version release of Checka11y.css and not in the current version release.
• I confirm the work in this pull request is valid according to my findings and is not something for anything personal.
• I have updated the README and/or features.md where and if applicable (still put an x if you have considered this but thought there was nothing to add or modify).
• I have added myself to the contributors section in package.json (still put an x if you have considered this but decided not to add yourself).
• I have checked I have not committed any accidental files.
• I have tested all the main modern browsers (I.e. Chrome, Firefox, Edge, Safari - please leave this unchecked if there were any browsers listed you could not test and list them in the help section with details why you couldn't test that browser)
• I have run the automated tests and added new ones to cover new code.
• All new and existing a11y checks still work correctly (compare your local test/index.html to the test/index.html in the master branch).

Help

I could not test in Safari because I do not have access to an Apple device.

[jackdomleo7]

Nice job here! Just one minor thing I'd like adding 🙂

[jackdomleo7]

Thank you for this PR!