This should make future maintenance easier since Gulp v4 has better
documentation [1], more checks for incorrect usage and a simpler API for
expressing what should run in parallel vs sequentially during build
steps. This also eliminates a bunch of old transitive dependencies that
gulp v3 had, which sometimes caused problems (eg. with Node upgrades).

Changes required:

 - Make sure `gulp.task` functions return a value, so Gulp knows when
   the task is finished
 - `gulp.watch` now requires a function rather than a task name
 - Task no longer support a list of dependencies but need a task
   function. Dependencies are expressed using `gulp.series` or
   `gulp.parallel` to compose other functions
 - Give several functions names so that there are fewer `<anonymous>
   {starting, finished}` messages in log output.

[1] https://gulpjs.com/docs/en/api/concepts