feat(ivy): accept multiple values for exportAs in the compiler (#28001)

exportAs in @Directive metadata supports multiple values, separated by
commas. Previously it was treated as a single value string.

This commit modifies the compiler to understand that exportAs is a
string[]. It stops short of carrying the multiple values through to the
runtime. Instead, it only emits the first one. A future commit will modify
the runtime to accept all the values.

PR Close #28001

packages/compiler-cli/src/ngtsc/annotations/src/directive.ts

@@ -177,7 +177,7 @@ export function extractDirectiveMetadata(
           member.name === 'ngOnChanges');
 
   // Parse exportAs.
-  let exportAs: string|null = null;
+  let exportAs: string[]|null = null;
   if (directive.has('exportAs')) {
     const expr = directive.get('exportAs') !;
     const resolved = evaluator.evaluate(expr);
@@ -185,7 +185,7 @@ export function extractDirectiveMetadata(
       throw new FatalDiagnosticError(
           ErrorCode.VALUE_HAS_WRONG_TYPE, expr, `exportAs must be a string`);
     }
-    exportAs = resolved;
+    exportAs = resolved.split(',').map(part => part.trim());
   }
 
   // Detect if the component inherits from another class

packages/compiler-cli/src/ngtsc/annotations/src/selector_scope.ts

@@ -366,7 +366,7 @@ export class SelectorScopeRegistry {
       name: clazz.name !.text,
       directive: ref,
       isComponent: def.name === 'ngComponentDef', selector,
-      exportAs: readStringType(def.type.typeArguments[2]),
+      exportAs: readStringArrayType(def.type.typeArguments[2]),
       inputs: readStringMapType(def.type.typeArguments[3]),
       outputs: readStringMapType(def.type.typeArguments[4]),
       queries: readStringArrayType(def.type.typeArguments[5]),