Add support for textDocument/foldingRange

[fbricon]

Add support for textDocument/foldingRange, which allows editors to provide folding support (for code blocks, imports, folding regions ...). See LSP proposal: https://github.com/Microsoft/vscode-languageserver-protocol-foldingprovider/blob/master/protocol.foldingProvider.md

[fbricon]

Requires upstream lsp4j implementation: eclipse-lsp4j/lsp4j#169

[fbricon]

I missed the missing region delimiters folding support, before merging #962.
@yaohaizh can you please add support for:

• //#region and //#endregion
• //region and //endregion
• Netbeans format : // <editor-fold desc="Description"> and // </editor-fold>

[yaohaizh]

Interesting. Should we support the //#region and //#endregion? It is a C# grammar.

[fbricon]

yup, just so that we keep feature parity with the current vscode-java implem