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/eval.c

@@ -7067,6 +7067,9 @@ char *set_cmdarg(exarg_T *eap, char *oldarg)
   if (eap->bad_char != 0) {
     len += 7 + 4;  // " ++bad=" + "keep" or "drop"
   }
+  if (eap->mkdir_p != 0) {
+    len += 4;
+  }
 
   const size_t newval_len = len + 1;
   char *newval = xmalloc(newval_len);
@@ -7100,6 +7103,11 @@ char *set_cmdarg(exarg_T *eap, char *oldarg)
     snprintf(newval + strlen(newval), newval_len, " ++bad=%c",
              eap->bad_char);
   }
+
+  if (eap->mkdir_p) {
+    snprintf(newval, newval_len, " ++p");
+  }
+
   vimvars[VV_CMDARG].vv_str = newval;
   return oldval;
 }