Problem:
If completeopt does not include "popup" flag, nvim__complete_set still
auto-creates a floating preview window.

Solution:
Fail if completeopt does not include the "popup" flag.

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`.

Problem:  When completion leader changes, text that might be reinserted
          immediately after is deleted. This unnecessarily affects
          extmarks. #31387 restored the original extmarks but that
          prevents end_right_gravity marks from growing.
Solution: Avoid deleting leader text that will be reinserted.

- move credits and backers to credits.txt

Also remove "silent" to be more consistent with Normal mode search.

Problem:
Built-in completion can still affect nearby extmarks. #31384

Solution:
Restore extmarks when completion leader changes.

Problem:
The default unimpaired mappings display an empty line after the
command's output. This results (with default configuration) in the
`Press ENTER or type command to continue` prompt to be displayed, like
so:
```
(2 of 16): item2

Press ENTER or type command to continue
```

Solution:
The cause is that we're checking the second return value from
`pcall(vim.api.nvim_cmd, opts, {})` to determine whether the call was
successful. `nvim_cmd` returns an empty string on success, so this value
is an empty string in the successful path which we then display.

The fix is simple: check the first return value instead which is the
"status code" of the call.

Problem: `[<Space>` and `]<Space>` do not support repetition.

Solution: use `operatorfunc` and `g@l` to make these mappings dot
repeatable.

Before calling "attach" a screen object is just a dummy container for
(row, col) values whose purpose is to be sent as part of the "attach"
function call anyway.

Just create the screen in an attached state directly. Keep the complete
(row, col, options) config together. It is still completely valid to
later detach and re-attach as needed, including to another session.