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/ui/messages_spec.lua

@@ -2062,7 +2062,7 @@ describe('ui/msg_puts_printf', function()
     )
 
     cmd = cmd .. '"' .. nvim_prog .. '" -u NONE -i NONE -Es -V1'
-    command([[call termopen(']] .. cmd .. [[')]])
+    command([[call jobstart(']] .. cmd .. [[',{'term':v:true})]])
     screen:expect([[
     ^Exモードに入ります。ノー |
     マルモードに戻るには "vis|