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/legacy/window_cmd_spec.lua

@@ -120,7 +120,7 @@ describe('splitkeep', function()
             row = 0,
             col = 0,
           }))
-          vim.cmd("call termopen([&sh, &shcf, 'true'], { 'on_exit': 'C2' })")
+          vim.cmd("call jobstart([&sh, &shcf, 'true'], { 'term': v:true, 'on_exit': 'C2' })")
       end
     })]])
     feed('j')