v1-toolkit is failing to recognize the gulpfile.js

**Description:**
I've updated gulp to V4, also the gulpfile.js.

But, the last commit is searching for plain functions, which is not a GulpV4 standard.
Anyway I've accomodated the file to accomplish what is expected, so the gulpfile.js is recognized and tasks loaded, but again is not working correctly with the promises.

**Steps to Reproduce:**
"ionic serve" is no longer working for v1 projects, with the last version of packages/@ionic/v1-toolkit/src/lib/gulp.ts ...
Even when the "gulp" command works correctly by itself.

Can you provide a small example of what is expected in the gulp file?

Why is validating for plain functions, but then it just executes "ionic:serve:before" task

Best!

Start with packages/@ionic/v1-toolkit/src/lib/gulp.ts and reproduce the failure with ionic serve in a v1 project using the reported gulpfile.js and Gulp V4. Check how the file is recognized, how ionic:serve:before is executed, and how promise-based tasks are handled. Done means the gulpfile is recognized and ionic serve works correctly for the reported setup.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.