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/api/buffer_updates_spec.lua

@@ -879,7 +879,8 @@ describe('API: buffer events:', function()
   it('when :terminal lines change', function()
     local buffer_lines = {}
     local expected_lines = {}
-    fn.termopen({ nvim_prog, '-u', 'NONE', '-i', 'NONE', '-n', '-c', 'set shortmess+=A' }, {
+    fn.jobstart({ nvim_prog, '-u', 'NONE', '-i', 'NONE', '-n', '-c', 'set shortmess+=A' }, {
+      term = true,
       env = { VIMRUNTIME = os.getenv('VIMRUNTIME') },
     })