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

runtime/doc/editing.txt

@@ -438,6 +438,8 @@ Where {optname} is one of:	    *++ff* *++enc* *++bin* *++nobin* *++edit*
     bad			    specifies behavior for bad characters
     edit		    for |:read| only: keep option values as if editing
 			    a file
+    p			    creates the parent directory (or directories) of
+			    a filename if they do not exist
 
 {value} cannot contain white space.  It can be any valid value for these
 options.  Examples: >