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`.

src/nvim/api/buffer.c

@@ -1181,12 +1181,12 @@ ArrayOf(Integer, 2) nvim_buf_get_mark(Buffer buffer, String name, Arena *arena,
 /// This temporarily switches current buffer to "buffer".
 /// If the current window already shows "buffer", the window is not switched.
 /// If a window inside the current tabpage (including a float) already shows the
-/// buffer, then one of these windows will be set as current window temporarily.
+/// buffer, then one of those windows will be set as current window temporarily.
 /// Otherwise a temporary scratch window (called the "autocmd window" for
 /// historical reasons) will be used.
 ///
 /// This is useful e.g. to call Vimscript functions that only work with the
-/// current buffer/window currently, like |termopen()|.
+/// current buffer/window currently, like `jobstart(…, {'term': v:true})`.
 ///
 /// @param buffer     Buffer handle, or 0 for current buffer
 /// @param fun        Function to call inside the buffer (currently Lua callable