add: export interface Item

[marius-turtle]

Description: Updating the documentation
for usage at the element-declarations in the class, otherwise error

Checklist

• Issue number for this PR: #nnn (required) -
• Docs included?: (yes/no; required for all API/functional changes)
• Test units included?: (yes/no; required)
• In a clean directory, yarn install, yarn test run successfully? (yes/no; required)

Description

Code sample

[EdricChan03]

Could you modify your commit to use a valid commit type?

Something like:

docs(firestore/documents): add missing interface

See the Angular Commit Guidelines for more info.

[jamesdaniels]

Thanks for the fix. I altered the commit message with a squash.

[marius-turtle]

Thank you. I am quite new at committing something on GitHub, next time I will use the template-format from the Angular Commit Guidelines.