Introduce (experimental) Context.VERSION_ECMASCRIPT

[p-bakker]

In preparation of moving towards by default running Rhino in an EcmaSscript compliant mode, we need a new Context.VERSION_ECMASCRIPT to be able to use in code to branch/enable/disable certain behaviors

For v1.7.15, if this version is set as the version to use on a Context, a BIG warning should be raised/logged/... that while the Context.VERSION_ECMASCRIPT could be used in v1.7.15, its purpose is to prepare for v2.0.0 and if already starting to use it in v1.7.15 there most likely will be breaking changes between v1.7.15 and v2.0.0