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/vim_spec.lua

@@ -2704,7 +2704,8 @@ describe('API', function()
       -- :terminal with args + running process.
       command('enew')
       local progpath_esc = eval('shellescape(v:progpath)')
-      fn.termopen(('%s -u NONE -i NONE'):format(progpath_esc), {
+      fn.jobstart(('%s -u NONE -i NONE'):format(progpath_esc), {
+        term = true,
         env = { VIMRUNTIME = os.getenv('VIMRUNTIME') },
       })
       eq(-1, eval('jobwait([&channel], 0)[0]')) -- Running?