Update Gulp to version 4, split gulpfile into individual tasks

[Nooshu]

Fairly simple PR to upgrade Gulp to version 4. Individual gulp tasks have been split out into separate files under the gulp-tasks directory for easier maintenance. Gulp 4 allows for the use of gulp.series() which simplifies the task running process and removing the need for task references nested inside other tasks. It is also possible to use gulp.parallel() if this is required.

A gulp-paths.js file has been created to store relevant paths outside of the individual tasks.

To install it should be a case of running npm i and version 4 will be installed. npm build is untouched.

[soulgalore]

Thank you @Nooshu !