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/editor/mode_normal_spec.lua

@@ -26,10 +26,10 @@ describe('Normal mode', function()
 
   it('&showcmd does not crash with :startinsert #28419', function()
     local screen = Screen.new(60, 17)
-    fn.termopen(
-      { n.nvim_prog, '--clean', '--cmd', 'startinsert' },
-      { env = { VIMRUNTIME = os.getenv('VIMRUNTIME') } }
-    )
+    fn.jobstart({ n.nvim_prog, '--clean', '--cmd', 'startinsert' }, {
+      term = true,
+      env = { VIMRUNTIME = os.getenv('VIMRUNTIME') },
+    })
     screen:expect({
       grid = [[
         ^                                                            |