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/src/render3/view/api.ts

@@ -104,7 +104,7 @@ export interface R3DirectiveMetadata {
    * Reference name under which to export the directive's type in a template,
    * if any.
    */
-  exportAs: string|null;
+  exportAs: string[]|null;
 
   /**
    * The list of providers defined in the directive.