#4607 No conflicts with nvim-lspconfig by default

Default `g:ale_disable_lsp` to a new mode `'auto'` by default. With this
setting applied, ALE will now check for the presence of nvim-lspconfig
and automatically turn off particular LSP linters if already configured
via nvim-lspconfig.

For users that do not use `nvim-lspconfig`, everything should work as
before.

lua/ale/util.lua

@@ -0,0 +1,14 @@
+local M = {}
+
+function M.configured_lspconfig_servers()
+    local configs = require 'lspconfig.configs'
+    local keys = {}
+
+    for key, _ in pairs(configs) do
+        table.insert(keys, key)
+    end
+
+    return keys
+end
+
+return M