nashorn script engine should support the usage multiple global objects with same engine instance

User should be able to create new global object using ScriptEngine.createBindings API.

ScriptEngine engine; // initialization skipped

Bindings newGlobal = engine.createBindings();
ScriptContext ctx = new ScriptContext();
ctx.setBindings(newGlobal, ScriptContext.ENGINE_SCOPE);


engine.eval("Object"); // refers to "Object" from default context's engine bindings
engine.eval("Object", ctx); // refers to "Object" from given context's engine bindings.

If engine_scope bindings of a ScriptContext is not nashorn engine created (by createBindings call), nashorn can fallback to use default global scope created for default context.

This essentially exposes nashorn's multiple-global-per-context feature (used in testruns) via standard API.