Update dependencies & upgrade gulpfile.js for Gulp 4

[markusdosch]

Thanks for providing this awesome landing page starter!

I tried to build this project (npm install && npx gulp build), but it didn't work (see screenshot attached). Probably it is because some of the dependencies are not compatible with the current npm LTS version.

In this course, I updated the project's dependencies with npm-check -u. This included an upgrade to Gulp 4. That is why I updated the gulpfile.js according to the best practices in the Gulp documentation.

The other changes are:

• Replaced rimraf with del, as this seems like the current way to go, see the deprecation of gulp-rimraf.
• Introduce the dependency to merge-stream to use multiple sources in one task, as documented in the gulp docs
• Move the browserslist specification into the package.json, as this is what autoprefixer recommends (when running gulp, autoprefixer emitted a warning that the specification should be moved to package.json).

Now, cloning the project and then npm install && npx gulp build successfully builds the project! 😄

[cssninjaStudio]

I'll test this pull request and merge it if it works properly. Thanks

[cssninjaStudio]

Closing this since I managed the update to gulp 4 and the template structure rework