Add a "JavaScript core (initial support)" feature

[Elchi3]

This is a feature that is a sink for all the basic JavaScript language features for which I believe we don't want to create separate features. They are all "baseline highest" meaning that baseline_low_date is always 2015-07-29 and baseline_high_date is always 2018-01-29 (the earliest possible dates currently).

[autonome]

A couple of questions / thoughts, none blocking:

• "basics" sounds like a tutorial or learning material, maybe something in the direction of "core" would better reflect that the intent?

• are you thinking prototype/functions as a separate feature? nm, saw the function PR

• a few builtins i could imagine breaking out into a types feature

• ditto for the addressing bits

Overall, +1 to the idea of a catchall for now though!

[captainbrosset]

I like this. I would have prefered it even more if the support data matched the first versions of browsers. But I don't see a compelling reason to remove debugger and empty from this feature to achieve this.

[ddbeck]

Sorry for getting hung up on names and descriptions here, but being a sink, I think it's important that we make sure we understand what we're telling ourselves here. A few ideas for you.

[Elchi3]

Thanks Daniel, I agree we want to make sure this sink feature is well described. I like your suggestions and updated this PR accordingly.

[ddbeck]

Cool. I'm pretty happy with this, at least as a first pass (before we get composition and such). Thank you!