feat: ngIf now supports else; saves condition to local var

NgIf syntax has been extended to support else clause to display template
when the condition is false. In addition the condition value can now
be stored in local variable, for later reuse. This is especially useful
when used with the `async` pipe.

Example:

```
<div *ngIf="userObservable | async; else loading; let user">
  Hello {{user.last}}, {{user.first}}!
</div>
<template #loading>Waiting...</template>
```

closes #13061
closes #13297

modules/@angular/language-service/test/test_data.ts

@@ -213,7 +213,7 @@ export class UnknownTrackBy {
     'ng-if-cases.ts': `
 import {Component} from '@angular/core';
 
-@Component({template: '<div ~{implicit}*ngIf="show; let l"~{implicit-end}>Showing now!</div>'})
+@Component({template: '<div ~{implicit}*ngIf="show; let l=unknown"~{implicit-end}>Showing now!</div>'})
 export class ShowIf {
   show = false;
 }

modules/@angular/language-service/test/ts_plugin_spec.ts

@@ -186,7 +186,7 @@ describe('plugin', () => {
         expectSemanticError('app/ng-if-cases.ts', locationMarker, message);
       }
       it('should report an implicit context reference', () => {
-        expectError('implicit', 'The template context does not have an implicit value');
+        expectError('implicit', "The template context does not defined a member called 'unknown'");
       });
     });
   });