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_defs.h

@@ -202,6 +202,7 @@ struct exarg {
   int regname;                  ///< register name (NUL if none)
   int force_bin;                ///< 0, FORCE_BIN or FORCE_NOBIN
   int read_edit;                ///< ++edit argument
+  int mkdir_p;                  ///< ++p argument
   int force_ff;                 ///< ++ff= argument (first char of argument)
   int force_enc;                ///< ++enc= argument (index in cmd[])
   int bad_char;                 ///< BAD_KEEP, BAD_DROP or replacement byte