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.c

@@ -71,6 +71,7 @@ int file_open(FileDescriptor *const ret_fp, const char *const fname, const int f
   FLAG(flags, kFileReadOnly, O_RDONLY, kFalse, wr != kTrue);
 #ifdef O_NOFOLLOW
   FLAG(flags, kFileNoSymlink, O_NOFOLLOW, kNone, true);
+  FLAG(flags, kFileMkDir, O_CREAT|O_WRONLY, kTrue, !(flags & kFileCreateOnly));
 #endif
 #undef FLAG
   // wr is used for kFileReadOnly flag, but on
@@ -78,6 +79,13 @@ int file_open(FileDescriptor *const ret_fp, const char *const fname, const int f
   // `error: variable ‘wr’ set but not used [-Werror=unused-but-set-variable]`
   (void)wr;
 
+  if (flags & kFileMkDir) {
+    int mkdir_ret = os_file_mkdir((char *)fname, 0755);
+    if (mkdir_ret < 0) {
+      return mkdir_ret;
+    }
+  }
+
   const int fd = os_open(fname, os_open_flags, mode);
 
   if (fd < 0) {