Have npm run typings use built directory; fix typescript tests

* Modify scripts/gulpfiles/typings.js to write typings to BUILD_DIR.
* Modify tests/scripts/compile_typings.sh to check compilability of
  resulting output from BUILD_DIR.
* Rename checkin script to checkin:built, add a checkin:typings script
  to do the same for .d.ts files, and a new checkin script to do both.
* Have recompile run checkin:typings.

scripts/gulpfiles/release_tasks.js

@@ -178,6 +178,7 @@ const recompile = gulp.series(
   buildTasks.build,
   buildTasks.checkinBuilt,
   typings.typings,
+  typings.checkinTypings,
   gitTasks.pushRebuildBranch
   );