Problem:    The screen.c file is much too big.
Solution:   Split it in three parts. (Yegappan Lakshmanan, closes vim/vim#4943)
https://github.com/vim/vim/commit/7528d1f6b5422750eb778dfb550cfd0b0e540964

This is an approximation vim-patch 8.1.2057. Applying the patch directly
isn't feasible since our version of screen.c has diverged too much,
however we still introduce drawscreen.c and drawline.c:
- screen.c is now a much smaller file used for low level screen functions
- drawline.c contains everything needed for win_line()
- drawscreen.c contains everything needed for update_screen()

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

- reimplement 'winhl' in terms of highlight namespaces
- check for EOF in screen tests (to indicate a likely crash)

docs(extmark): remove message about passing in id=0

The docs for `nvim_buf_set_extmark` mention that you can create a new
extmark when passing in `id=0`, however if you do this you'll get an
error since the code checks that id is positive.

```
id is not a positive integer
```

This change re-words the sentence to make it clearer.

refactor: replace char_u variables and functions with char

Work on https://github.com/neovim/neovim/issues/459