remove array and string usage from gulp.watch

gulp.watch('glob', ['task1', 'task2'])
gulp.watch('glob', 'task2')

will have to be changed to

gulp.watch('glob', gulp.parallel('task1', 'task2'))
gulp.watch('glob', gulp.task('task2'))

This makes it a lot clearer what is going on.

Lots of documentation is going to need to be updated.

Semi-related: I also think we should discourage use of a watch task and just have people do it like this

gulp.task('a', fn);
gulp.task('b', fn);

gulp.watch('glob', gulp.parallel('a', 'b'));

and if they have a flag for when to watch it can be as simple as

gulp.task('a', fn);
gulp.task('b', fn);

if (process.env.NODE_ENV !== 'production) {
  gulp.watch('glob', gulp.parallel('a', 'b'));
}

I feel like this would simplify a lot of things