Needs to enforce and document that global "context" and "engine" can't be modified when running via jsr223

When a script is evaluated thanks to ScriptEngine.eval, the evaluated script global acquires a "context" property. This property references a ScriptContext.

If this context is modified (e.g.: context = function doIt(){}) then the script execution is broken.

This should be clearly documented.

Testcase, print becomes undefined:

var se = new javax.script.ScriptEngineManager();
var e = se.getEngineByName("nashorn");
e.eval("print('context ' + context);context = function(){}; print('Never reached');");