feat(compiler): narrow types of expressions used in *ngIf (#20702)

Structural directives can now specify a type guard that describes
what types can be inferred for an input expression inside the
directive's template.

NgIf was modified to declare an input guard on ngIf.

After this change, `fullTemplateTypeCheck` will infer that
usage of `ngIf` expression inside it's template is truthy.

For example, if a component has a property `person?: Person`
and a template of `<div *ngIf="person"> {{person.name}} </div>`
the compiler will no longer report that `person` might be null or
undefined.

The template compiler will generate code similar to,

```
  if (NgIf.ngIfTypeGuard(instance.person)) {
    instance.person.name
  }
```

to validate the template's use of the interpolation expression.
Calling the type guard in this fashion allows TypeScript to infer
that `person` is non-null.

Fixes: #19756?

PR Close #20702

packages/compiler/src/core.ts

@@ -51,6 +51,7 @@ export interface Directive {
   providers?: Provider[];
   exportAs?: string;
   queries?: {[key: string]: any};
+  guards?: {[key: string]: any};
 }
 export const createDirective =
     makeMetadataFactory<Directive>('Directive', (dir: Directive = {}) => dir);