Execute synchronous code in the cypress command execution pipeline

`cy.then` is not the same as a promise then, so don't use it with [async/await](https://github.com/cypress-io/cypress/issues/1417).

The key usecase for `then` is to inject synchronous code *into* the cypress command execution pipeline.

e.g.

```ts
cy.something()
cy.then(()=>{
// This code will only execute after something is done.
})
```

Start by reading the linked Cypress issue #1417 and the cy.then example in this issue. Confirm whether the requested change belongs in this TypeScript book or upstream in Cypress; the issue does not name a repository file, test, or acceptance criteria, so those need to be established before work can begin.

Written by the indexing model from the issue text.