feat(editor): ":restart" command #33953

Problem:
Developing/troubleshooting plugins has friction because "restarting"
Nvim requires quitting and manually starting again. #32484

Solution:
- Implement a `:restart` command which emits `restart` UI event.
- Handle the `restart` UI event in the builtin TUI client: stop the
  `nvim --embed` server, start a new one, and attach to it.

src/nvim/ex_cmds.lua

@@ -3368,6 +3368,12 @@ M.cmds = {
     addr_type = 'ADDR_LINES',
     func = 'ex_substitute',
   },
+  {
+    command = 'restart',
+    flags = bit.bor(BANG, FILES, CMDARG, ARGOPT, TRLBAR, CMDWIN, LOCK_OK),
+    addr_type = 'ADDR_NONE',
+    func = 'ex_restart',
+  },
   -- commands that start with an uppercase letter
   {
     command = 'Next',