fix(language-service): Suggest ? and ! operator on nullable receiver (#35200)

Under strict mode, the language service fails to typecheck nullable
symbols that have already been verified to be non-null.

This generates incorrect (false positive) and confusing diagnostics
for users.

To work around this issue in the short term, this commit changes the
diagnostic message from an error to a suggestion, and prompts users to
use the safe navigation operator (?) or non-null assertion operator (!).

For example, instead of

```typescript
{{ optional && optional.toString() }}
```

the following is cleaner:

```typescript
{{ optional?.toString() }}
{{ optional!.toString() }}
```

Note that with this change, users who legitimately make a typo in their
code will no longer see an error. I think this is acceptable, since
false positive is worse than false negative. However, if users follow
the suggestion, add ? or ! to their code, then the error will be surfaced.
This seems a reasonable trade-off.

References:

1. Safe navigation operator (?)
   https://angular.io/guide/template-syntax#the-safe-navigation-operator----and-null-property-paths
2. Non-null assertion operator (!)
   https://angular.io/guide/template-syntax#the-non-null-assertion-operator---

PR closes https://github.com/angular/angular/pull/35070
PR closes https://github.com/angular/vscode-ng-language-service/issues/589

PR Close #35200

packages/language-service/test/project/app/parsing-cases.ts

@@ -154,6 +154,7 @@ export class TemplateReference {
   heroesByName: {[name: string]: Hero} = {};
   primitiveIndexType: {[name: string]: string} = {};
   anyValue: any;
+  optional?: string;
   myClick(event: any) {}
 }