feat(terminal): surface exit code via virttext + nvim_get_chan_info #37987

Problem:
When a terminal process exits, "[Process Exited]" text is added
to the buffer contents.

Solution:
- Return `exitcode` field from `nvim_get_chan_info`.
- Show it in the default 'statusline'.
- Show exitcode as virtual text in the terminal buffer.

runtime/lua/vim/_meta/api.lua

@@ -1309,6 +1309,7 @@ function vim.api.nvim_get_autocmds(opts) end
 --- -  "buffer"  (optional) Buffer connected to |terminal| instance.
 --- -  "client"  (optional) Info about the peer (client on the other end of the channel), as set
 ---              by |nvim_set_client_info()|.
+--- - "exitcode" (optional) Exit code of the |terminal| process.
 ---
 function vim.api.nvim_get_chan_info(chan) end