Start up language servers with Neovim's API

Get language servers starting and displaying diagnostics with Neovim's
API in Neovim 0.8 and up. With this set up, now ALE needs to take over
handling diagnostics returned by the language servers.

lua/ale/lsp.lua

@@ -0,0 +1,62 @@
+local module = {}
+
+vim.lsp.set_log_level("debug")
+
+module.start = function(config)
+    -- Neovim's luaeval sometimes adds a Boolean key to table we need to remove.
+    if config.init_options[true] ~= nil then
+        config.init_options[true] = nil
+    end
+
+    config.on_init = function(_, _)
+        vim.defer_fn(function()
+            vim.fn["ale#lsp#CallInitCallbacks"](config.name)
+        end, 0)
+    end
+
+    return vim.lsp.start(config, {
+        attach = false,
+        silent = true,
+    })
+end
+
+module.buf_attach = function(args)
+    return vim.lsp.buf_attach_client(args.bufnr, args.client_id)
+end
+
+module.buf_detach = function(args)
+    return vim.lsp.buf_detach_client(args.bufnr, args.client_id)
+end
+
+-- Send a message to an LSP server.
+-- Notifications do not need to be handled.
+--
+-- Returns -1 when a message is sent, but no response is expected
+--         0 when the message is not sent and
+--         >= 1 with the message ID when a response is expected.
+module.send_message = function(args)
+    local client = vim.lsp.get_client_by_id(args.client_id)
+
+    if args.is_notification then
+        local success = client.notify(args.method, args.params)
+
+        if success then
+            return -1
+        end
+
+        return 0
+    end
+
+    -- NOTE: We aren't yet handling reponses to requests properly!
+    -- NOTE: There is a fourth argument for a bufnr here, and it's not
+    --       clear what that argument is for or why we need it.
+    local success, request_id = client.request(args.method, args.params)
+
+    if success then
+        return request_id
+    end
+
+    return 0
+end
+
+return module