Codechange: Simplify FioCreateDirectory.

[PeterN]

Motivation / Problem

FioCreateDirectory is a recursive function that attempts to create a directory tree if it does not exist, and uses platform-specific functions to do so.

Description

std::filesystem provides create_directories() as a cross-platform way to create a directory tree with one call.

Limitations

Has not been tested on Windows (with or without unicode paths.)

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker