Ensure defineElement is used instead of customElement

[vsilvar]

Description

The custom defineElement decorator is now used instead of the default lit customElement.
Also adds an ESLint rule configured as a warning, to try to prevent this in the future.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (minor updates related to the tooling or maintenance of the repository, does not impact compiled assets)

Motivation and context

The custom defineElement decorator prevents elements from being registered multiple times.
This is partially related to umbraco/Umbraco-CMS#15397.

Checklist

• If my change requires a change to the documentation, I have updated the documentation in this pull request.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[github-actions]

Hi there @vsilvar, thank you for this contribution! 👍

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• The automated tests all pass (see "Checks" tab on this PR)
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[iOvergaard]

Good catch, thanks. H5YR!