Use CompletionItemLabelDetails from LSP4J 0.20.0 for detailed completion list

[trustytrojan]

• Bump LSP4J to 0.20.0
• Remove org.eclipse.lsp4j.jsonrpc as an explicit dependency as to let org.eclipse.lsp4j itself require the version of org.eclipse.lsp4j.jsonrpc it needs
• Use CompletionItemLabelDetails object to set detail to the argument list of methods and description to the return type of methods and the types of properties/fields. This allows for a nicer looking completion menu showing much more information about methods/properties/fields at a glance in VS Code. (This also brings the completion menu to parity with Eclipse JDT LS.)
• Modify test cases to check CompletionItemLabelDetails objects
• Use CompletionItemLabelDetails for class/type completions to show the package of each class as the description field. The package name shows up in the same place that method/field/variable types do.

Visual demonstration

With the following class available in scope, below are screenshots of the completion menu after typing new TestClass().:

class TestClass {
    int aInt = 3
    byte aByte = 4
    String aString = "hello"
    abstract def aMethod()
    abstract float anotherMethod(short a, double b)
    abstract List aThirdMethod(x)
}

Before

After