Add a isCategory method to Webform entity

If the webform module can leverage this API, we should consider adding it. I do not see any immediate use case.

For me, it provides assistance to a website where a custom module modifies Webforms depending on their category. For example, I modify the attributes and add tooltips to submit, preview, next, previous buttons in a multi-page Webform. Previously, I could just check the single-value "category" attribute of a Webform, but now I was having to get the array of categories and check for the presence of the category I want. Using new isCategory() method, I am able to standardize this check for whether a Webform belongs to a certain category.

For custom code, I find helper functions/methods always useful.

Yep, me too, so I am using that for my site. Thought this kind of one would help the community. Happy either way.

Let's move this to won't fix unless the webform module or other contrib modules need this API.