feat: ":write ++p" creates parent dirs #20835

- `:write ++p foo/bar/baz.txt` should create parent directories `foo/bar/` if
   they do not exist
    - Note: `:foo ++…` is usually for options. No existing options have
      a single-char abbreviation (presumably by design), so it's safe to
      special-case `++p` here.
- Same for `writefile(…, 'foo/bar/baz.txt', 'p')`
- `BufWriteCmd` can see the ++p flag via `v:cmdarg`.

closes #19884

src/nvim/os/fileio.h

@@ -35,6 +35,7 @@ typedef enum {
                      ///< be used with kFileCreateOnly.
   kFileNonBlocking = 128,  ///< Do not restart read() or write() syscall if
                            ///< EAGAIN was encountered.
+  kFileMkDir = 256,
 } FileOpenFlags;
 
 static inline bool file_eof(const FileDescriptor *fp)