refactor(compiler-cli): use a transformer for dts files (#28342)

The current DtsFileTransformer works by intercepting file writes and editing the source string directly.

This PR refactors it as a afterDeclaration transform in order to fit better in the TypeScript API.

This is part of a greater effort of converting ngtsc to be usable as a TS transform plugin.

PR Close #28342

packages/compiler-cli/src/ngtsc/transform/index.ts

@@ -8,5 +8,5 @@
 
 export * from './src/api';
 export {IvyCompilation} from './src/compilation';
-export {DtsFileTransformer} from './src/declaration';
+export {DtsFileTransformer, declarationTransformFactory} from './src/declaration';
 export {ivyTransformFactory} from './src/transform';