Use `cy.should` for the assertions

[wKich]

In Cypress there is cy.should command https://docs.cypress.io/api/commands/should that created especially to make assertions. Currently, we execute all interactions in cy.then context. So technically it doesn't matter how to write tests:

cy.do([
  TextField('User name').fillIn('John Foo'),
  Checkbox('Remember me').is({ checked: true }),
  Button('Submit').click()
])

Or

cy.expect([
  TextField('User name').fillIn('John Foo'),
  Checkbox('Remember me').is({ checked: true }),
  Button('Submit').click()
])

Or

cy.do(TextField('User name').fillIn('John Foo'))
cy.expect(Checkbox('Remember me').is({ checked: true }))
cy.do(Button('Submit').click())

All of them become like this:

cy.then(() => TextField('User name').fillIn('John Foo'))
cy.then(() => Checkbox('Remember me').is({ checked: true }))
cy.then(() => Button('Submit').click())

And also I think, there is no reason to check the runner state and restrict calling actions after assertions. Because users always will have a workaround to do it.