Add Stylelint task

[yguedidi]

Q	A
Branch	master
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Documented?	yes
Fixed tickets

Add a task to support Stylelint tool. Heavily inspired from the existing ESLint task. Supports relevant CLI supported arguments, based on the --help output, the inspiration for the sorting. Parameter description is comming from the official documentation

Extra commit for a small CS fix in ESLint test.

New Task Checklist:

• [x] Are the dependencies added to the composer.json suggestions?
• [x] Is the doc/tasks.md file updated?
• [x] Are the task parameters documented?
• [x] Is the task registered in the tasks.yml file?
• [x] Does the task contains phpunit tests?
• [x] Is the configuration having logical allowed types?
• [x] Does the task run in the correct context?
• [x] Is the run() method readable?
• [x] Is the run() method using the configuration correctly?
• [x] Are all CI services returning green?

[yguedidi]

@veewee hello, any missing thing here? if any don't hesitate to tell me!

[yguedidi]

@veewee updated with fixup commits, tell me if it's OK for you so I squash them before merge

[veewee]

Looks good, thanks for the PR and sorry for the long time of silence in between :)

[yguedidi]

@veewee it's a pleasure! if I knew you planned to merge it, I would have squashed the fixup commit, for a cleaner git history :)