Create + document formal API's around cy internal properties

[brian-mann]

• [ ] Rename the internal cy.prop to cy.state and move store to a private property.
• [ ] Rename the internal cy.private to cy.privateState and move store to a private property.
• [ ] Throw a very specific error message when accessing the old properties directly (which was never supported or documented)
• [ ] Throwing on cy.props or cy.privates should tell the user to go through cy.state(...) or cy.privateState(...)
• [ ] Document cy.state and why you'd use it https://github.com/cypress-io/cypress-documentation/issues/108

[jennifer-shehane]

@chrisbreiding @brian-mann Can either of you confirm:

1. Was this work ever done?
2. Does this work still need to be done?

[proCrichard]

Any progress here? Need information on cy.state(). Specifically how to add objects/images to test context for use with reporters like Mochawesome.

[andrey-semin]

+1

[jennifer-shehane]

Closing since there hasn't been activity here in a while. Refer to issue open in documentation for cy.state() work: https://github.com/cypress-io/cypress-documentation/issues/108