Define what worker_support means

[saschanaz]

Worker support is worker support, yes, but there are multiple types of workers and thing are not always available on all of them.

I think worker_support should indicate all worker support, and if needed, smaller pieces like sharedworker_support be introduced. I think existing worker_support items generally already means that. What do you think?

See also: #7849

[queengooborg]

I think we've considered the worker_support subfeature to imply support in any worker type, but I don't think we've documented this nor actually decided whether this is the case. This is definitely a useful point to document!

[foolip]

I think it should mean "supported in the worker types that the spec requires" and that we should use partial_implementation if support is less than that.

An exception is if a kind of worker just didn't exist at one point, it doesn't make sense to make everything partial_support because of lack of service worker support before the point that service workers existed.