fix(vim.ui): open() may wait indefinitely #28325

Problem:
vim.ui.open "locks up" Nvim if the spawned process does not terminate. #27986

Solution:
- Change `vim.ui.open()`:
    - Do not call `wait()`.
    - Return a `SystemObj`. The caller can decide if it wants to `wait()`.
- Change `gx` to `wait()` only a short time.
    - Allows `gx` to show a message if the command fails, without the
      risk of waiting forever.

runtime/lua/vim/_system.lua

@@ -18,6 +18,7 @@ local uv = vim.uv
 --- @field stderr? string
 
 --- @class vim.SystemState
+--- @field cmd string[]
 --- @field handle? uv.uv_process_t
 --- @field timer?  uv.uv_timer_t
 --- @field pid? integer
@@ -56,6 +57,7 @@ local function close_handles(state)
 end
 
 --- @class vim.SystemObj
+--- @field cmd string[]
 --- @field pid integer
 --- @field private _state vim.SystemState
 --- @field wait fun(self: vim.SystemObj, timeout?: integer): vim.SystemCompleted
@@ -68,6 +70,7 @@ local SystemObj = {}
 --- @return vim.SystemObj
 local function new_systemobj(state)
   return setmetatable({
+    cmd = state.cmd,
     pid = state.pid,
     _state = state,
   }, { __index = SystemObj })