replace EOF with std::char_traits<char>::eof()

Fix typo in README

Fixes #2730

Add documentation for numbers

JSON_VERSION_IS_PREDEFINED with JSON_HAS_CPP_11

Add support for deserialization of STL containers of non-default constructable types (fixes #2574).

Use Clang 12 in CI

Update json.hpp

Added define JSON_VERSION_IS_PREDEFINED
for skipping automatic JSON_HAS_CPP_* detection

Proposed fix for #2706

pkg-config.pc.in: Don't concatenate paths

See: https://github.com/jtojnar/cmake-snips#assuming-cmake_install_dir-is-relative-path

add more CI steps