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`.

src/nvim/eval.c

@@ -8493,7 +8493,7 @@ char *do_string_sub(char *str, size_t len, char *pat, char *sub, typval_T *expr,
   return ret;
 }
 
-/// common code for getting job callbacks for jobstart, termopen and rpcstart
+/// Common code for getting job callbacks for `jobstart`.
 ///
 /// @return true/false on success/failure.
 bool common_job_callbacks(dict_T *vopts, CallbackReader *on_stdout, CallbackReader *on_stderr,