fix(helpers): add missing tests and docs for helpers plugin"

[matteo-cristino]

• test(helpers): add missing tests for merge and omit
• docs: add hlpers examples

[puria]

hmmm too many .license files maybe like we do in zenroom https://github.com/dyne/Zenroom/blob/master/.reuse/dep5 e can add a /**.keys etc etc in place of all this license files ;)

[matteo-cristino]

@puria I'm not totally sure how dep5 file works, is it correct how I did?

[puria]

@matteo-cristino you can install reuse with:

pipx install reuse

and run the linter

reuse lint

a occhio nudo seems that the format is correct ;p