Codechange: Ensure SDLK mappings stay in the expected order.

[PeterN]

Motivation / Problem

#12596 found an issue with SDLK mappings that we did not know about, but we can make the compiler tell us.

Description

Add a constexpr constructor that ensures at compile-time that the source SDLK range matches the target range.

Therefore, it is no longer possible (fingers-crossed...) for the key ranges to be incorrect.

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

[rubidium42]

Something similar can also be done for Allegro and Windows.
For MacOS the mapping is fully 1:1.