Check for differing neovim versions with different APIs for LSP.
Fixes#2324
Consists of two tests:
1. Before extension load, test if this is neovim and whether this neovim
supports LSP
2. When getting diagnostic counts, test whether a language server is
attached to the current buffer
closes#2324
Note: seems like one needs to call v:lua.vim.lsp.buf_get_clients() and
even then some older neovims do not understand that. So wrap it around
an exists() call and hope this works for all neovims.
nvimlsp removed the :LspInstallInfo command. So instead check explicitly
for neovim for enabling the nvimlsp extension.
In the extension itself check that at least one LSP is attached to the
buffer before returning warning and error messages.
closes#2323
