refactor(i18n): extract Extractor from extract_i18n (#12417)

I put an extractor into your extract so you can extract while you
extract.

This allows integrators to call Extractor as a library. Also refactors
Extractor a bit so that callers need fewer arguments or arguments that
are at the right semantic level.

The refactoring causes no function change.

modules/@angular/compiler-cli/index.ts

@@ -7,6 +7,7 @@
  */
 
 export {CodeGenerator} from './src/codegen';
+export {Extractor} from './src/extractor';
 export {NodeReflectorHostContext, ReflectorHost, ReflectorHostContext} from './src/reflector_host';
 export {StaticReflector, StaticReflectorHost, StaticSymbol} from './src/static_reflector';