Deprecate @JSExportNamed

[sjrd]

@JSExportNamed has funny semantics, and I find it very fragile in terms of language design. Their signature significantly differ on the Scala and JavaScript sides. It is also very partial, as it only supports the case where all parameters are passed in one config object (so no way for one param to be positional and the others to be named, for example).

It also seems not to be used. Google reports 0 use of @JSExportNamed, while GitHub reports 2 uses:

• here with a call here
• here with a call here

The ratio benefit/cost seems to be extremely low for this feature.

I suggest we deprecate it, and remove it in 1.0.0. Any opposition?