refactor typedefs for cy

cli/types/index.d.ts

@@ -4237,6 +4237,8 @@ declare namespace Cypress {
   // Diff taken from https://github.com/Microsoft/TypeScript/issues/12215#issuecomment-311923766
   type Diff<T extends string, U extends string> = ({ [P in T]: P } & { [P in U]: never } & { [x: string]: never })[T]
   type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>
+
+interface cy extends Chainable<undefined> {}
 }
 
 /**
@@ -4248,7 +4250,7 @@ cy.get('button').click()
 cy.get('.result').contains('Expected text')
 ```
  */
-declare const cy: Cypress.Chainable<undefined>
+declare const cy: Cypress.cy
 /**
  * Global variable `Cypress` holds common utilities and constants.
  * @see https://on.cypress.io/api

cli/types/tests/plugins-test.ts

@@ -0,0 +1,27 @@
+/**
+ * Test that plugin authors can write .d.ts files to extend Cypress types
+ * Unfortunately we cannot test that vendor types located in node_modules are working
+ * since those are copied as part of the deploy process
+ *
+ */
+
+declare namespace Cypress {
+  interface cy {
+    cyProp: number
+	}
+	interface Chainable {
+    chainerProp: number
+  }
+}
+
+// $ExpectType number
+cy.cyProp
+
+// $ExpectError
+cy.get('.element').cyProp
+
+// $ExpectType number
+cy.chainerProp
+
+// $ExpectType number
+cy.get('.element').chainerProp