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/terminal/cursor_spec.lua

@@ -239,7 +239,7 @@ describe(':terminal cursor', function()
     feed([[<C-\><C-N>]])
     command('set statusline=~~~')
     command('new')
-    call('termopen', { testprg('tty-test') })
+    call('jobstart', { testprg('tty-test') }, { term = true })
     feed('i')
     screen:expect({
       grid = [[