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`.

runtime/lua/vim/_defaults.lua

@@ -435,7 +435,7 @@ do
     group = nvim_terminal_augroup,
     desc = 'Treat term:// buffers as terminal buffers',
     nested = true,
-    command = "if !exists('b:term_title')|call termopen(matchstr(expand(\"<amatch>\"), '\\c\\mterm://\\%(.\\{-}//\\%(\\d\\+:\\)\\?\\)\\?\\zs.*'), {'cwd': expand(get(matchlist(expand(\"<amatch>\"), '\\c\\mterm://\\(.\\{-}\\)//'), 1, ''))})",
+    command = "if !exists('b:term_title')|call jobstart(matchstr(expand(\"<amatch>\"), '\\c\\mterm://\\%(.\\{-}//\\%(\\d\\+:\\)\\?\\)\\?\\zs.*'), {'term': v:true, 'cwd': expand(get(matchlist(expand(\"<amatch>\"), '\\c\\mterm://\\(.\\{-}\\)//'), 1, ''))})",
   })
 
   vim.api.nvim_create_autocmd({ 'TermClose' }, {