bug: mockdoc lacks `part` api implementation

[johnjenkins]

Prerequisites

• I have read the Contributing Guidelines.
• I agree to follow the Code of Conduct.
• I have searched for existing issues that already report this problem, without success.

Stencil Version

latest

Current Behavior

Stencil's mockdoc (used during internal spec tests and SSR) lacks an implementation of the part DOM api.

Expected Behavior

The part dom api should be implemented in mockdoc. It has a very similar (the same?) api to the classList api - which has already been implemented - so should be quite straightforward

System Info

Steps to Reproduce

Run this.element.part.add('something') and get Cannot read properties of undefined (reading 'add')

Code Reproduction URL

any

Additional Information

No response