Introduce nvim namespace: Move files.

Move files from src/ to src/nvim/.
- src/nvim/ becomes the new root dir for nvim executable sources.
- src/libnvim/ is planned to become root dir of the neovim library.

src/nvim/version.h

@@ -0,0 +1,12 @@
+#ifndef NEOVIM_VERSION_H
+#define NEOVIM_VERSION_H
+
+int highest_patch(void);
+int has_patch(int n);
+void ex_version(exarg_T *eap);
+void list_version(void);
+void maybe_intro_message(void);
+void intro_message(int colon);
+void ex_intro(exarg_T *eap);
+
+#endif  // NEOVIM_VERSION_H