Language Server: implement textDocument/definition , textDocument/declaration, textDocument/references

[hzeller]

This will require the project set-up to know which are all the relevant files to pull symbol information from #1188

For this, we then need to build a symbol table (and fix issues in project-tool Issues related to the project tool/symbol table ) to have all the relevant information available to know how to find definitions and references of a particular symbol, which is then trivial to implement.

[hzeller]

textDocument/definition implemented in #1530