fix(common): narrow NgIf context variables in template type checker (#36627)

When the `NgIf` directive is used in a template, its context variables
can be used to capture the bound value. This is sometimes used in
complex expressions, where the resulting value is captured in a
context variable. There's two syntax forms available:

1. Binding to `NgIfContext.ngIf` using the `as` syntax:
```html
<span *ngIf="enabled && user as u">{{u.name}}</span>
```

2. Binding to `NgIfContext.$implicit` using the `let` syntax:
```html
<span *ngIf="enabled && user; let u">{{u.name}}</span>
```

Because of the semantics of `ngIf`, it is known that the captured
context variable is truthy, however the template type checker
would not consider them as such and still report errors when
`strict` is enabled.

This commit updates `NgIf`'s context guard to make the types of the
context variables truthy, avoiding the issue.

Based on https://github.com/angular/angular/pull/35125

PR Close #36627

goldens/public-api/common/common.d.ts

@@ -243,7 +243,7 @@ export declare class NgIf<T = unknown> {
     set ngIfThen(templateRef: TemplateRef<NgIfContext<T>> | null);
     constructor(_viewContainer: ViewContainerRef, templateRef: TemplateRef<NgIfContext<T>>);
     static ngTemplateGuard_ngIf: 'binding';
-    static ngTemplateContextGuard<T>(dir: NgIf<T>, ctx: any): ctx is NgIfContext<NonNullable<T>>;
+    static ngTemplateContextGuard<T>(dir: NgIf<T>, ctx: any): ctx is NgIfContext<Exclude<T, false | 0 | '' | null | undefined>>;
 }
 
 export declare class NgIfContext<T = unknown> {