vim-patch:8.1.2045: the option.c file is too big (#19854)

Problem:    The option.c file is too big.
Solution:   Split off the code dealing with strings. (Yegappan Lakshmanan,
            closes vim/vim#4937)
dac1347b4d

Cherry-pick set_string_option_direct_in_win() from patch 8.1.1405.
Cherry-pick shift_line() comment change from patch 8.1.2096.
Move 'clipboard' default parsing to didset_string_options().
Reorder option flags to put Nvim-only flags at the end.

src/nvim/buffer.c

@@ -66,6 +66,7 @@
 #include "nvim/message.h"
 #include "nvim/move.h"
 #include "nvim/option.h"
+#include "nvim/optionstr.h"
 #include "nvim/os/input.h"
 #include "nvim/os/os.h"
 #include "nvim/os/time.h"