[structure view] create project structure view UI for VSCode

[martinlippert]

Create a tree view part for VSCode that can display the structural view of a Spring project.

In order to get the project tree nodes, we need a new custom LSP message to get this information from the language server, which can deliver this information using the SpringMetamodelIndex nodes for projects.

At the moment, the outline view (based on LSP DocumentSymbols) uses document symbols created from this project structure, but it transfers objects of type DocumentSymbol between the client and the server. Theoretically, we could do the same for the whole project, but we need to decide whether it is better to transfer the index nodes directly as a data structure or let the server create a document symbol tree from it and transfer that to the client.