Add support of the official workspaceFolders

[fortime]

Currently, JDT Language Server uses initializationOptions to pass workspaceFolders.
https://github.com/eclipse/eclipse.jdt.ls/blob/1dfdeeafb9d0dd7c16d915763a30088dfa3a721e/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/handlers/InitHandler.java#L93

Since 3.6, lsp has added a parameter for passing workspaceFolders.
https://github.com/eclipse/lsp4j/blob/2a3db10fe9719f3869a715c58fd683bd493dfdeb/org.eclipse.lsp4j/src/main/xtend-gen/org/eclipse/lsp4j/InitializeParams.java#L73-L81

I use ycmd, it uses the official parameter.
Please add support for it. Maybe I can create a PR. But, I am not clear if I should keep the old way.

[joaotavora]

In the interim, something that is probably easiest to implement is to allow the code that @fortime points to to accept not only a JSON vector of strings, but also a JSON vector of objects containing "uri" and "name" as keys. Then at least it will be closer to what the LSP protocol specifies and allow clients to more easily adapt (like here joaotavora/eglot#909 (reply in thread))