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/scrollback_spec.lua

@@ -355,7 +355,9 @@ describe(':terminal prints more lines than the screen height and exits', functio
   it('will push extra lines to scrollback', function()
     clear()
     local screen = Screen.new(30, 7, { rgb = false })
-    command(("call termopen(['%s', '10']) | startinsert"):format(testprg('tty-test')))
+    command(
+      ("call jobstart(['%s', '10'], {'term':v:true}) | startinsert"):format(testprg('tty-test'))
+    )
     screen:expect([[
       line6                         |
       line7                         |
@@ -623,7 +625,7 @@ describe('pending scrollback line handling', function()
       local bufnr = vim.api.nvim_create_buf(false, true)
       local args = ...
       vim.api.nvim_buf_call(bufnr, function()
-        vim.fn.termopen(args)
+        vim.fn.jobstart(args, { term = true })
       end)
       vim.api.nvim_win_set_buf(0, bufnr)
       vim.cmd('startinsert')