fix: Adjust breadcrumb position logic

[herzog31]

Description

• This PR allows the position of an item in BreadcrumbList to be a non-numeric string to be compliant with https://schema.org/position.
• This change required the logic to determine the last item in the list to change:
  • Determining the last item in the list is only possible, if all position values are either numbers or strings that can be parsed as number. The last item is not required to have an item attribute.
  • If just a single position value is a string, no last element can be determined and every list item is required to have an item attribute with a valid URL.
  • This behaviour is consistent with the Google Rich Results validator.

How Has This Been Tested?

• Added new unit test and verified existing tests still pass.

Screenshots (if appropriate):

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)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[github-actions]

🎉 This PR is included in version 1.2.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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