feat(ui): UI :detach command

Problem:
Cannot detach the current UI.

Solution:
- Introduce `:detach`.
- Introduce `Channel.detach`.

Co-authored-by: bfredl <bjorn.linse@gmail.com>

src/nvim/ex_cmds.lua

@@ -732,6 +732,12 @@ M.cmds = {
     addr_type = 'ADDR_NONE',
     func = 'ex_delfunction',
   },
+  {
+    command = 'detach',
+    flags = bit.bor(BANG, FILES, CMDARG, ARGOPT, TRLBAR, CMDWIN, LOCK_OK),
+    addr_type = 'ADDR_NONE',
+    func = 'ex_detach',
+  },
   {
     command = 'display',
     flags = bit.bor(EXTRA, NOTRLCOM, TRLBAR, SBOXOK, CMDWIN, LOCK_OK),