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`.
When libvterm receives the OSC 52 escape sequence it ignores it because
Nvim does not set any selection callbacks. Install selection callbacks
that forward to the clipboard provider, so that setting the clipboard
with OSC 52 in the embedded terminal writes to the system clipboard
using the configured clipboard provider.
