Merge responses from multiple servers

[garyo]

At the moment, it only tries to merge completion responses. The basic merging structure is there, but more work is needed.

Methods that need merging:

• textDocument/completion (merge completion lists)
• textDocument/diagnostic (merge diagnostic lists)
• textDocument/hover (combine hover texts)
• textDocument/signatureHelp (combine signatures)
• textDocument/documentSymbol (merge symbol lists)
• textDocument/codeAction (merge action lists)
• textDocument/references (merge reference locations)
• textDocument/definition (merge definition locations)
• textDocument/documentHighlight (merge highlights)
• textDocument/foldingRange (merge folding ranges)

Methods that are notifications (no response needed):

• textDocument/didOpen
• textDocument/didClose
• textDocument/didChange
• textDocument/didSave

Methods that might need special handling:

• textDocument/formatting (might need to be sequential - format with one server then the next)
• textDocument/rename (might need coordination between servers)