docs(util): added jsdoc annotations for ArrayHelpers.js

[taranek]

Hi there, it's my first contribution to this repo, so I tried to find something easy to make my debut smoother. I hope I won't break contribution guidelines🤞

What kind of change does this PR introduce?

Adds missing JSDoc type annotations for util/ArrayHelpers.js file.
Improves type safety of groupBy and equals util methods by providing an optional type parameter representing the type of a single array element (which defaults to any).

Did you add tests for your changes?

No.

Does this PR introduce a breaking change?

No.

What needs to be documented once your changes are merged?

Nothing - it's just a small improvement of the existing documentation.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Tomasz Taranek (43b60ab, 3cf3f49)

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[sokra]

Thanks