Stopping an LSP makes the editor unusable

[WhyNotHugo]

Looks like the on_attach function sets a CursorHold handler, which gets triggered even after running LspStop.

The consequence is that if an LSP is stopped, neovim will continuously spam the following error:

Error detected while processing CursorHold Autocommands for "<buffer=1>":
method textDocument/documentSymbol is not supported by any of the servers registered for the current buffer

I think there may be two possible fixes:

1. Remove the CursorHold handler when an LSP detatches. This seems like the correct fix, but I'm not sure how to do it.
2. Check that an LSP supports textDocument/documentSymbol inside update_current_function. This might be more inefficient but easier to do.