Ensure correct checking of LSP support on neovim

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

autoload/airline/extensions.vim

@@ -338,7 +338,8 @@ function! airline#extensions#load()
   endif
 
   if (get(g:, 'airline#extensions#nvimlsp#enabled', 1)
-        \ && has("nvim"))
+        \ && has('nvim')
+        \ && luaeval('vim.lsp ~= nil'))
     call airline#extensions#nvimlsp#init(s:ext)
     call add(s:loaded_ext, 'nvimlsp')
   endif