Use Clang 12 in CI

Update json.hpp

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

Update doctest from 2.4.4 to 2.4.6 (fixes #2686)

License fix

Properly select "Release" build for Travis

# Conflicts:
#	.travis.yml

* ⚗ move CI targets to CMake
* ♻ add target for cpplint
* ♻ add target for self-contained binaries
* ♻ add targets for iwyu and infer
* 🔊 add version output
* ♻ add target for oclint
* 🚨 fix warnings
* ♻ rename targets
* ♻ use iwyu properly
* 🚨 fix warnings
* ♻ use iwyu properly
* ♻ add target for benchmarks
* ♻ add target for CMake flags
* 👷 use GitHub Actions
* ⚗ try to install Clang 11
* ⚗ try to install GCC 11
* ⚗ try to install Clang 11
* ⚗ try to install GCC 11
* ⚗ add clang analyze target
* 🔥 remove Google Benchmark
* ⬆ Google Benchmark 1.5.2
* 🔥 use fetchcontent
* 🐧 add target to download a Linux version of CMake
* 🔨 fix dependency
* 🚨 fix includes
* 🚨 fix comment
* 🔧 adjust flags for GCC 11.0.0 20210110 (experimental)
* 🐳 user Docker image to run CI
* 🔧 add target for Valgrind
* 👷 add target for Valgrind tests
* ⚗ add Dart
* ⏪ remove Dart
* ⚗ do not call ctest in test subdirectory
* ⚗ download test data explicitly
* ⚗ only execute Valgrind tests
* ⚗ fix labels
* 🔥 remove unneeded jobs
* 🔨 cleanup
* 🐛 fix OCLint call
* ✅ add targets for offline and git-independent tests
* ✅ add targets for C++ language versions and reproducible tests
* 🔨 clean up
* 👷 add CI steps for cppcheck and cpplint
* 🚨 fix warnings from Clang-Tidy
* 👷 add CI steps for Clang-Tidy
* 🚨 fix warnings
* 🔧 select proper binary
* 🚨 fix warnings
* 🚨 suppress some unhelpful warnings
* 🚨 fix warnings
* 🎨 fix format
* 🚨 fix warnings
* 👷 add CI steps for Sanitizers
* 🚨 fix warnings
* ⚡ add optimization to sanitizer build
* 🚨 fix warnings
* 🚨 add missing header
* 🚨 fix warnings
* 👷 add CI step for coverage
* 👷 add CI steps for disabled exceptions and implicit conversions
* 🚨 fix warnings
* 👷 add CI steps for checking indentation
* 🐛 fix variable use
* 💚 fix build
* ➖ remove CircleCI
* 👷 add CI step for diagnostics
* 🚨 fix warning
* 🔥 clean Travis

Remove HEDLEY annotation from exception::what()

Update parse_exceptions.md - correct `json::exception::parse_error`

Fix amount of entries in the json object

Referring to https://github.com/nlohmann/json/blob/develop/doc/examples/parse_error.cpp  and https://json.nlohmann.me/home/exceptions/
(and also based on testing), the catch command should reference `catch (json::parse_error& ex)` and not `catch (json::exception::parse_error& ex)`

The latest MSVC compiler throws the following warning on
nlohmann::detail::exception::what() if /analyze is enabled:

```
vcruntime_exception.h(93) : warning C28204: 'what' has an override at
`nlohmann\json\develop\single_include\nlohmann\json.hpp(2644)`
and only the override is annotated for return: when an override is
annotated, the base (this function) should be similarly annotated.
```

See https://godbolt.org/z/r331h4

After the initial j.push_back() calls there is another j.emplace_back() call that makes the size == 4 not 3.

Better diagnostics

 Conflicts:
	include/nlohmann/detail/input/parser.hpp
	single_include/nlohmann/json.hpp

Fix missing 1.78 in example in README.md