feat(common): support as syntax in template/* bindings (#15025)

* feat(common): support `as` syntax in template/* bindings

Closes #15020

Showing the new and the equivalent old syntax.
- `*ngIf="exp as var1”`
   => `*ngIf="exp; let var1 = ngIf”`
- `*ngFor="var item of itemsStream |async as items”`
   => `*ngFor="var item of itemsStream |async; let items = ngForOf”`

* feat(common): convert ngIf to use `*ngIf="exp as local“` syntax

* feat(common): convert ngForOf to use `*ngFor=“let i of exp as local“` syntax

* feat(common): expose NgForOfContext and NgIfContext

tools/public_api_guard/common/typings/common.d.ts

@@ -151,13 +151,26 @@ export declare const NgFor: typeof NgForOf;
 /** @stable */
 export declare class NgForOf<T> implements DoCheck, OnChanges {
     ngForOf: NgIterable<T>;
-    ngForTemplate: TemplateRef<NgForOfRow<T>>;
+    ngForTemplate: TemplateRef<NgForOfContext<T>>;
     ngForTrackBy: TrackByFunction<T>;
-    constructor(_viewContainer: ViewContainerRef, _template: TemplateRef<NgForOfRow<T>>, _differs: IterableDiffers);
+    constructor(_viewContainer: ViewContainerRef, _template: TemplateRef<NgForOfContext<T>>, _differs: IterableDiffers);
     ngDoCheck(): void;
     ngOnChanges(changes: SimpleChanges): void;
 }
 
+/** @stable */
+export declare class NgForOfContext<T> {
+    $implicit: T;
+    count: number;
+    readonly even: boolean;
+    readonly first: boolean;
+    index: number;
+    readonly last: boolean;
+    ngForOf: NgIterable<T>;
+    readonly odd: boolean;
+    constructor($implicit: T, ngForOf: NgIterable<T>, index: number, count: number);
+}
+
 /** @stable */
 export declare class NgIf {
     ngIf: any;
@@ -166,6 +179,12 @@ export declare class NgIf {
     constructor(_viewContainer: ViewContainerRef, templateRef: TemplateRef<NgIfContext>);
 }
 
+/** @stable */
+export declare class NgIfContext {
+    $implicit: any;
+    ngIf: any;
+}
+
 /** @experimental */
 export declare class NgLocaleLocalization extends NgLocalization {
     protected locale: string;