Introduce BeyondJavaScriptEngine.

BeyondJavaScriptEngine is responsible for execution of JavaScript.
Make GamePluginWorker take an instance of BeyondJavaScriptEngine.
Extract initialization of BeyondGlobal as init method.
Make it possible to pass a BeyondGlobal instance to BeyondJavaScriptEngine.
Add BeyondShellGlobal.

BeyondShellGlobal is a subclass of BeyondGlobal which provide additional
features for shell console. Currently, only quit is added.
Add JavaScriptShellConsole.

JavaScriptShellConsole is an interactive JavaScript shell.
Make it possible to set an ErrorReporter to BeyondContextFactory.
Use TotalErrorReporter in JavaScriptShellConsole.

TotalErrorReporter intercepts exceptions and prints them to stderr.
Add tab auto-completion to JavaScriptShellConsole.