Don't start the language server until VSCode indicates that the file is opened in the editor

[bobbrow]

VSCode sends a textDocument/didOpen message in cases besides when a document is made visible in the editor. In these cases we end up spawning an entire language server just to shut it down again. CTL+hover is one such case. VSCode does not intend to update the LSP for this, so we need to do something else.

We should consider adding logic to confirm a file is actually open in the editor before starting the language server. This can be either by checking the visibleEditors or by detecting some other LSP request.