Add ability to compile with a different JDK than the one used to run jdt.ls

[fbricon]

We need to make it possible to use a JDK for project compilation/debugging, that is different from the JDK version used to actually run jdt.ls. So you could compile a Java 12 project but running jdt.ls with JDK 8, or debugging a project with JDK 8, when jdt.ls has been started with JDK 12. You'd receive the proper sources matching the execution environment of the project.

One possible approach would be to define JDT execution environments in the server preferences as:

java.environments{
  "JavaSE-1.8": {
      path: "/path/to/jdk.1.8",
      sources: "/optional/path/to/sources",
      javadoc: "uri/or/optional/path/to/javadoc"
  },
   "JavaSE-12": { 
      path: "/path/to/jdk.12"
  }
}

[snjeza]

A duplicate of #1307