set stepper mat icon code example

Example: mat stepper custom icon edit

<mat-vertical-stepper>
  <ng-template matStepperIcon="edit">
    <mat-icon>insert_drive_file</mat-icon>
  </ng-template>

  <ng-template matStepperIcon="done">
    <mat-icon>done_all</mat-icon>
  </ng-template>

  <!-- Custom icon with a context variable. -->
  <ng-template matStepperIcon="number" let-index="index">
    {{index + 10}}
  </ng-template>

  <!-- Stepper steps go here -->
</mat-vertical-stepper>