Deprecate "language level" concept

[gbrail]

Getting ready to release 1.8.0 soon.

How do people feel about deprecating the "languageVersion" concept? That would mean deprecating the "setLanguageVersion" flag in Context, sending a message that people should be using the default language level (now "VERSION_ECMASCRIPT") and stop relying on Rhino being able to support parts of the JavaScript spec that have changed in ECMAScript, or that Rhino never implemented properly in the first place.

This would at least put us on a path to getting rid of this stuff.

OTOH, there are a LOT of tests that only run on older language levels, so either we don't need those any more (yay) or they need to be updated.