Codechange: Use std::optional for town parent scope resolver.

[PeterN]

Motivation / Problem

When resolving NewGRF, the parent town_scope is lazily initialised as it does not always need to be used.

But it uses a raw pointer so requires manual management with new/delete. Airports tried to use a unique_ptr, but still used new instead of make_unique.

Description

As TownScopeResolver is not large, replace the manually managed pointer with std::optional to simplify it.

Using std::optional also avoids extra memory allocation.

Limitations

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