feat(jobs): jobstart(…,{term=true}), deprecate termopen() #31343

Problem:
`termopen` has long been a superficial wrapper around `jobstart`, and
has no real purpose. Also, `vim.system` and `nvim_open_term` presumably
will replace all features of `jobstart` and `termopen`, so centralizing
the logic will help with that.

Solution:
- Introduce `eval/deprecated.c`, where all deprecated eval funcs will live.
- Introduce "term" flag of `jobstart`.
- Deprecate `termopen`.

test/functional/testterm.lua

@@ -141,7 +141,7 @@ function M.setup_screen(extra_rows, cmd, cols, env, screen_opts)
   })
 
   api.nvim_command('enew')
-  api.nvim_call_function('termopen', { cmd, env and { env = env } or nil })
+  api.nvim_call_function('jobstart', { cmd, { term = true, env = (env and env or nil) } })
   api.nvim_input('<CR>')
   local vim_errmsg = api.nvim_eval('v:errmsg')
   if vim_errmsg and '' ~= vim_errmsg then