feat(stepper): add state input to step

feat: added error state in order to display the correct icon and a default state to display any custom state/icon

refactor: add StepState type

refactor: refactor getIndicatorType to become more readable

feat: add stepper example using a custom state

feat: add error step theme

feat: update getIndicatorType logic to handle a custom state

feat: add alertMessage input for the step and conditionally apply error css

feat: add icon override demo example and allow any icon to be overridden

fix: issue with icon colors when selected or not

test: update/add unit tests for the stepper changes

test: update e2e tests for the stepper changes

docs(stepper): update the readme to include new functionality

refactor: add missing comment

refactor: use mixins for stepper theme error

Author: arodr967

src/cdk/stepper/stepper.ts

@@ -110,6 +110,9 @@ export class CdkStep implements OnChanges {
    */
   @Input('aria-labelledby') ariaLabelledby: string;
 
+  /** Alert message when there's an error. */
+  @Input() alertMessage: string;
+
   /** Whether the user can return to this step once it has been marked as complted. */
   @Input()
   get editable(): boolean { return this._editable; }