Switch from grunt to gulp for more flexible build

[pavelhoral]

This is mainly about personal preference, but there are also few minor advantages to changing from Grunt to Gulp:

• from the two task runners, Gulp is definitely the more modern one (even though Gulp can feel a bit rusty compared to other hype tools, we can't get completely rid of generic task runner)
• Gulp supports modern day JS much much better (mainly talking about ESM and async API)
• it is far easier to write clean and compact custom tasks with Gulp than with Grunt (e.g. when task configuration has to be dynamically constructed)
• I am not a huge fan of Grunt's codebase (large single-procedure scripts)
• I would like to move Maven's responsibilities to JS world (i.e. get rid of maven-resources-plugin)
• task runners are not playing that important role in the project and are not shared so we can switch without any major impact

[karelmaxa]

Double slash is intentional?

[karelmaxa]

LGTM.