Javascript artifact

* Create a **pom.xml** which defines the "Tool" portion of the JavaScript target, along with its tests
* Extract a common base class `CommonBaseTest`
* Provide a default value when the `antlr-javascript-runtime` property is not specified (defaults to the `src` directory in this target's repository)
* Use node.js binaries from Maven when available for the current platform, so tests run in most cases even if `antlr-javascript-nodejs` is not specified