Structural directives

Only one may be present on an element.

if: *ngIf
for: *ngFor
- the minimum: let tag of tags
- extras: let tag of tags; index as i; trackBy: tag.slug; (index, first, last, even, odd)
- with observable and async pipe: let user of usersObservable | async as users
switch: [ngSwitch] + *ngSwitchCase + *ngSwitchDefault