Issue: SPR-12266

This commit adds support for script based templating. Any templating
library running on top of a JSR-223 ScriptEngine that implements
Invocable like Nashorn or JRuby could be used.

For example, in order to render Mustache templates thanks to the Nashorn
Javascript engine provided with Java 8+, you should declare the following
configuration:

@Configuration
@EnableWebMvc
public class MustacheConfig extends WebMvcConfigurerAdapter {

	@Override
	public void configureViewResolvers(ViewResolverRegistry registry) {
		registry.scriptTemplate();
	}

	@Bean
	public ScriptTemplateConfigurer configurer() {
		ScriptTemplateConfigurer configurer = new ScriptTemplateConfigurer();
		configurer.setEngineName("nashorn");
		configurer.setScripts("mustache.js");
		configurer.setRenderObject("Mustache");
		configurer.setRenderFunction("render");
		return configurer;
	}
}

The XML counterpart is:

<beans>
	<mvc:annotation-driven />

	<mvc:view-resolvers>
		<mvc:script-template />
	</mvc:view-resolvers>

	<mvc:script-template-configurer engine-name="nashorn" render-object="Mustache" render-function="render">
		<mvc:script location="mustache.js" />
	</mvc:script-template-configurer>
</beans>

Tested with:
 - Handlebars running on Nashorn
 - Mustache running on Nashorn
 - React running on Nashorn
 - EJS running on Nashorn
 - ERB running on JRuby
 - String templates running on Jython

Issue: SPR-12266