refactor: trimmed down virtual key test

chore: update Appveyor for VS 2022.

Update to C++20 (#312)

* feat: Terminal++ now builds for C++20

* fix: static constexpr variables are now inline constexpr

* feat: colours use <=> and fixup noexcept

* feat: effects use <=> and fixup noexcept

* feat: attributes use <=> and fixup noexcept

* feat: glyphs use <=> and fixup noexcept

* feat: character sets use <=> and fixup noexcept

* feat: elements use <=> and fixup noexcept

* feat: extents use <=> and fixup noexcept

* feat: points use <=> and fixup noexcept

* feat: rectangles use <=> and fixup noexcept

* feat: string uses <=>

* refactor: gsl-span is now std-span; removed gsl-lite

* refactor: apply concepts to for_each_in_region

* refactor: series of refactorings

* chore: upgrade google test version

* fix: potential uninitialized access

* fix: update gcov version

Add explicit initialization of extender during parsing

Install libfmt from apt (#303)

* Install libfmt from apt
* Added gsl-lite as public dependency of Terminal++

Terminal++ requires gsl-lite 0.38 (#302)

* Terminal++ requires gsl-lite 0.38
* Fixed UDL test.

Improved the terminal interface (#301)

* Improved interface of terminals.

Replaced read/write continuations with a channel concept, which
can match up with those from Console++ and Telnet++ with little
effort.  This reduces the complexity in the interface even further.

Terminal++ now requires C++17 (#298)

* Terminal++ now requires C++17.

* Replaced boost::optional with std::optional.
* Replaced boost::string_view with std::string_view.
* Uses of boost::variant are now std::variant.
* Converted colour details from union to variant.

Updated examples and readme with new screen API.

Fixed ability to create Conan packages.

This was accidentally broken by the changes that picked up generated
version numbers from git.  This change fixes that.