Add Neovim TCP connections to language servers

Support TCP connections to language servers through Neovim's built in
client. In all but what is currently the nightly builds of Neovim
connections via a hostname will fail, but connections via an IP address
should function. We will still enable the built in Neovim client by
default anyway, as LSP clients very rarely connect over TCP.

autoload/ale/lsp.vim

@@ -569,6 +569,21 @@ function! ale#lsp#StartProgram(conn_id, executable, command) abort
     return l:job_id > 0
 endfunction
 
+" Split an address into [host, port].
+" The port will either be a number or v:null.
+function! ale#lsp#SplitAddress(address) abort
+    let l:port_match = matchlist(a:address, '\v:(\d+)$')
+
+    if !empty(l:port_match)
+        let l:host = a:address[:-len(l:port_match[1]) - 2]
+        let l:port = l:port_match[1] + 0
+
+        return [l:host, l:port ? l:port : v:null]
+    endif
+
+    return [a:address, v:null]
+endfunction
+
 " Connect to an LSP server via TCP.
 "
 " 1 will be returned if the connection is running, or 0 if the connection could
@@ -577,8 +592,23 @@ function! ale#lsp#ConnectToAddress(conn_id, address) abort
     let l:conn = s:connections[a:conn_id]
     let l:started = 0
 
-    " TODO: Start Neovim client here.
-    if !has_key(l:conn, 'channel_id') || !ale#socket#IsOpen(l:conn.channel_id)
+    if g:ale_use_neovim_lsp_api && !l:conn.is_tsserver
+        let [l:host, l:port] = ale#lsp#SplitAddress(a:address)
+
+        let l:client_id = luaeval('require("ale.lsp").start(_A)', {
+        \   'name': a:conn_id,
+        \   'host': l:host,
+        \   'port': l:port,
+        \   'root_dir': l:conn.root,
+        \   'init_options': l:conn.init_options,
+        \})
+
+        if l:client_id > 0
+            let l:conn.client_id = l:client_id
+        endif
+
+        return l:client_id > 0
+    elseif !has_key(l:conn, 'channel_id') || !ale#socket#IsOpen(l:conn.channel_id)
         let l:channel_id = ale#socket#Open(a:address, {
         \   'callback': {_, mess -> ale#lsp#HandleMessage(a:conn_id, mess)},
         \})