Replace deprecated Nashorn scripting implementation

[outofcoffee]

Nashorn, the provider of JavaScript scripting support is deprecated as of Java 11.

The GraalVM implementation is intended to eventually replace the Nashorn implementation.

[outofcoffee]

GraalVM module added in 028875657848a71a16fd25caf05cf26f6e04a9a2.

[outofcoffee]

As of v1.3.0, the GraalVM implementation is now merged, but Nashorn is still the default.

[pcornish]

Currently Graal.js doesn't have special Map or List accessor syntax: https://github.com/oracle/graaljs/issues/107

This makes accessing things like params more cumbersome:

Nashorn:

request.headers.Authorization
request.params.foo

Graal.js

request.headers.get('Authorization')
request.params.get('foo')

[outofcoffee]

Graal.js now supports map-like data structures as of v21.1.0

Added support for map-like data structures (issue #143) ... Java host interoperability was improved to handle java.util.Map implementations to make them look like guest language hash maps. This feature can be enabled and disabled with HostAccess.Builder.allowMapAccess(boolean).

[outofcoffee]

Bumped Graal version in https://github.com/outofcoffee/imposter/commit/899487b4fc3ccce328e770dee2087726b315716c to support the above.

Next step is enabling native compiler support to avoid using interpreted mode, per: https://www.graalvm.org/reference-manual/js/RunOnJDK/

This will likely require packaging changes and a possible move to JDK11 to reduce the number of build targets.