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/optionstr.h

@@ -0,0 +1,10 @@
+#ifndef NVIM_OPTIONSTR_H
+#define NVIM_OPTIONSTR_H
+
+#include "nvim/buffer_defs.h"  // for buf_T, win_T
+#include "nvim/option_defs.h"
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "optionstr.h.generated.h"
+#endif
+#endif  // NVIM_OPTIONSTR_H