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`.

runtime/lua/vim/_meta/vimfn.lua

@@ -4805,7 +4805,7 @@ function vim.fn.jobresize(job, width, height) end
 --- @return any
 function vim.fn.jobsend(...) end
 
---- Note: Prefer |vim.system()| in Lua (unless using the `pty` option).
+--- Note: Prefer |vim.system()| in Lua (unless using `rpc`, `pty`, or `term`).
 ---
 --- Spawns {cmd} as a job.
 --- If {cmd} is a List it runs directly (no 'shell').
@@ -4813,8 +4813,11 @@ function vim.fn.jobsend(...) end
 ---   call jobstart(split(&shell) + split(&shellcmdflag) + ['{cmd}'])
 --- <(See |shell-unquoting| for details.)
 ---
---- Example: >vim
----   call jobstart('nvim -h', {'on_stdout':{j,d,e->append(line('.'),d)}})
+--- Example: start a job and handle its output: >vim
+---   call jobstart(['nvim', '-h'], {'on_stdout':{j,d,e->append(line('.'),d)}})
+--- <
+--- Example: start a job in a |terminal| connected to the current buffer: >vim
+---   call jobstart(['nvim', '-h'], {'term':v:true})
 --- <
 --- Returns |job-id| on success, 0 on invalid arguments (or job
 --- table is full), -1 if {cmd}[0] or 'shell' is not executable.
@@ -4879,6 +4882,10 @@ function vim.fn.jobsend(...) end
 ---   stdin:      (string) Either "pipe" (default) to connect the
 ---         job's stdin to a channel or "null" to disconnect
 ---         stdin.
+---   term:      (boolean) Spawns {cmd} in a new pseudo-terminal session
+---           connected to the current (unmodified) buffer. Implies "pty".
+---           Default "height" and "width" are set to the current window
+---           dimensions. |jobstart()|. Defaults $TERM to "xterm-256color".
 ---   width:      (number) Width of the `pty` terminal.
 ---
 --- {opts} is passed as |self| dictionary to the callback; the
@@ -10168,19 +10175,8 @@ function vim.fn.tanh(expr) end
 --- @return string
 function vim.fn.tempname() end
 
---- Spawns {cmd} in a new pseudo-terminal session connected
---- to the current (unmodified) buffer. Parameters and behavior
---- are the same as |jobstart()| except "pty", "width", "height",
---- and "TERM" are ignored: "height" and "width" are taken from
---- the current window. Note that termopen() implies a "pty" arg
---- to jobstart(), and thus has the implications documented at
---- |jobstart()|.
----
---- Returns the same values as jobstart().
----
---- Terminal environment is initialized as in |jobstart-env|,
---- except $TERM is set to "xterm-256color". Full behavior is
---- described in |terminal|.
+--- @deprecated
+--- Use |jobstart()| with `{term: v:true}` instead.
 ---
 --- @param cmd string|string[]
 --- @param opts? table