crossplatform definition

[JeromeSchmied]

which platforms does tui.hpp compile on, and with which compilers

I'd say the following ones should be tested: platform:

• windows: 7, 10, 11
• macos
• linux
• i(pad)os: ish
• android: termux

compiler:

• g++/gcc
• clang
• msvc

[JeromeSchmied]

added #23 for this purpose, but that's only supposed to check whether stuff build, not the logic part, and only yet includes windows and ubuntu