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/ex_terminal_spec.lua

@@ -175,7 +175,7 @@ local function test_terminal_with_fake_shell(backslash)
     api.nvim_set_option_value('shellxquote', '', {}) -- win: avoid extra quotes
   end)
 
-  it('with no argument, acts like termopen()', function()
+  it('with no argument, acts like jobstart(…,{term=true})', function()
     command('autocmd! nvim_terminal TermClose')
     feed_command('terminal')
     screen:expect([[
@@ -196,7 +196,7 @@ local function test_terminal_with_fake_shell(backslash)
     ]])
   end)
 
-  it("with no argument, but 'shell' has arguments, acts like termopen()", function()
+  it("with no argument, but 'shell' has arguments, acts like jobstart(…,{term=true})", function()
     api.nvim_set_option_value('shell', shell_path .. ' INTERACT', {})
     feed_command('terminal')
     screen:expect([[