Use default member initializer, constexpr for constructors and equality
operators and <, [[nodiscard]] for comparison operators.
Add simple unit tests for Selection. Not trying to be exhaustive here, just
start the process of adding tests.

tests but hidden from most of Scintilla.
Access through std::unique_ptr.

Replace [v]sprintf with bounds checked [v]snprintf.

in the margin or in the text.

Avoids some warnings but drops constexpr.
Use MixedWith in PlatWin for GDI instead of local implementation.
Add unit tests for Geometry.

Drop lexlib from include path as no longer part of Scintilla.
For clang on Windows, turn off deprecated function warning and disable
sanitizers as they are slow.