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/core/main_spec.lua

@@ -103,7 +103,8 @@ describe('command-line option', function()
 
       -- Need to explicitly pipe to stdin so that the embedded Nvim instance doesn't try to read
       -- data from the terminal #18181
-      fn.termopen(string.format([[echo "" | %s]], table.concat(args, ' ')), {
+      fn.jobstart(string.format([[echo "" | %s]], table.concat(args, ' ')), {
+        term = true,
         env = { VIMRUNTIME = os.getenv('VIMRUNTIME') },
       })
       screen:expect(