refactor(misc1): move out high-level input functions to a new file: input.c

Possibly dialog code is messages.c could be moved here as well.
misc1.c is now empty, so delete it.

src/nvim/input.h

@@ -0,0 +1,9 @@
+#ifndef NVIM_INPUT_H
+#define NVIM_INPUT_H
+
+#include "nvim/vim.h"
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "input.h.generated.h"
+#endif
+#endif  // NVIM_INPUT_H