Update to release/v8.1 and add cmake support

[nscooling]

Updates repo to align with release/v8.1

• changed lv_examples to lv_demos
• set submodule to use v8.1 branch
• add cmake support
• make build support Linux/gcc and macOS/AppleClang

Test on macOS and Ubuntu

[nscooling]

The latest update replaces the use of submodules with CMake FetchContent as covered in the Lvgl CMake documentation