Add example to cy.stub() for stubbing window.prompt behavior

[jennifer-shehane]

cy.visit('http://localhost:3000')
cy.window().then((win) => {
   cy.stub(win, 'prompt').returns('what i want typed into it')
})

[richdouglasevans]

Can this issue be closed?

Also, good work Team Cypress on really digging into that first timers only drive 👍

[srinivasrk]

Does it handle prompts generated anywhere in the page ?