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/ex_cmds.c

@@ -1922,6 +1922,13 @@ int do_write(exarg_T *eap)
       fname = curbuf->b_sfname;
     }
 
+    if (eap->mkdir_p) {
+      if (os_file_mkdir(fname, 0755) < 0) {
+        retval = FAIL;
+        goto theend;
+      }
+    }
+
     name_was_missing = curbuf->b_ffname == NULL;
     retval = buf_write(curbuf, ffname, fname, eap->line1, eap->line2,
                        eap, eap->append, eap->forceit, true, false);