feat: allow specifying docPath via settings when specifying classPath

[mozzieongit]

Hi, I'm using the java-language-server in projects without maven or gradle and being able to specify the classpath is very helpful. This also adds the ability to specify the docpath to add the corresponding sources so that the language server can present the docs included.

The settings would then include the following:

{
  "java": {
    "classPath": [
      "path-to-library.jar"
    ],
    "docPath": [
      "path-to-libarary-sources.jar"
    ]
  }
}