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_docmd.c

@@ -4074,6 +4074,13 @@ static int getargopt(exarg_T *eap)
     return OK;
   }
 
+  // ":write ++p foo/bar/file
+  if (strncmp(arg, "p", 1) == 0) {
+    eap->mkdir_p = true;
+    eap->arg = skipwhite(arg + 1);
+    return OK;
+  }
+
   if (STRNCMP(arg, "ff", 2) == 0) {
     arg += 2;
     pp = &eap->force_ff;