LSP4J is entangled with IDE

Currently xtext.ide.server extensively uses LSP4J which makes it hard to get dependencies right.
I will be using this example PR as basis for the explanation.

I wanted to create an adapter from IOccurrenceComputer to IDocumentHighlightService so that I only have to implement it once for it to work on LSP and Eclipse. The code is quite simple, just transform the inputs and the outputs.

The problem is that the IDocumentHighlightService API uses LSP4J arguments (TextDocumentPositionParams as input and DocumentHighlightKind as output) which requires to add LSP4J as a dependency to xtext.ui.

Another solution would be to do the transformations on the xtext.ide side, but that would require a dependency from xtext.ide to xtext.ui which is impossible.

In conclusion, there is no way to write an adapter from xtext.ui to xtext.ide without adding LSP4J as a dependency for xtext.ui. Creating new interfaces for the LSP4J arguments in xtext.ide and delegating those to said LSP4J objects could alleviate the problem.

Start with example PR #710, then trace how xtext.ide.server, xtext.ide, and xtext.ui use IOccurrenceComputer and IDocumentHighlightService. Compare the LSP4J input and output types and document the dependency constraints; done requires an agreed design that permits the adapter without adding the prohibited dependency.