expose isCy ("publicly") in docs and typescript defs

[shlomiassaf]

Current behavior:

cy,isCy is exposed physically at runtime but it's not documented and it does not appear is the typescript type definitions.

Desired behavior:

cy.isCy should be exposed in the docs and in TS typings so we know it's officially public and safe to use.

I found that cypress-pipe is using it and perhaps others as well...

Typescript signature should be something like this:

    interface Chainable {
      isCy<TSubject = any>(obj: Chainable<TSubject>): obj is Chainable<TSubject>;
      isCy(obj: any): obj is Chainable<any>;
    }

This will allow typescript to infer TSubject when possible while allowing non Chainable types to passthrough without complaints.

[flotwig]

This should be exposed, but it should be exposed on Cypress.isCy(), since it doesn't return a chainable like all the other cy. commands do.

[cypress-bot]

The code for this is done in cypress-io/cypress#4170, but has yet to be released.
We'll update this issue and reference the changelog when it's released.

[cypress-bot]

Released in 3.3.0.