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/title_spec.lua

@@ -80,7 +80,7 @@ describe('title', function()
   it('is updated in Terminal mode', function()
     api.nvim_set_option_value('title', true, {})
     api.nvim_set_option_value('titlestring', '(%{mode(1)}) | nvim', {})
-    fn.termopen({ n.testprg('shell-test'), 'INTERACT' })
+    fn.jobstart({ n.testprg('shell-test'), 'INTERACT' }, { term = true })
     screen:expect(function()
       eq('(nt) | nvim', screen.title)
     end)