Add SDL3_ttf unit

[Free-Pascal-meets-SDL-Website]

Adds

• SDL3_ttf.pas
• SDL_textengine.inc

[suve]

The C headers for SDL_ttf do not include SDL_textengine.h by themselves; if anything, it's SDL_textengine.h that contains #include <SDL3/SDL_ttf.h>. This effectively turns the TextEngine stuff into a separate, optional API on top of SDL_ttf.

I think our units should work similarly - instead of {$INCLUDE sdl_textengine.inc} in SDL3_ttf.pas, turn TextEngine into a proper, separate unit that may then use SDL3_ttf.pas.

[Free-Pascal-meets-SDL-Website]

@suve I changed it into a unit according to your suggestion. If this is ok with you, please merge.

Best regards