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/terminal.c

@@ -357,7 +357,7 @@ static void term_output_callback(const char *s, size_t len, void *user_data)
 
 /// Initializes terminal properties, and triggers TermOpen.
 ///
-/// The PTY process (TerminalOptions.data) was already started by termopen(),
+/// The PTY process (TerminalOptions.data) was already started by jobstart(),
 /// via ex_terminal() or the term:// BufReadCmd.
 ///
 /// @param buf Buffer used for presentation of the terminal.