Merge branches 'develop' and 'external_test_data' of https://github.com/nlohmann/json into external_test_data

Fix README.md. Missing ```

About pull request:
https://github.com/nlohmann/json/pull/2074

An error has been introduced by accepting the suggestions:
https://github.com/ArthurSonzogni/json/commit/4be4a038ccd17aab0a4cc85200a6a0b41e1cccf2

One was about removing ~~~, but it was meant to be replaced by ``` in
reality. This caused the README.md to be slightly broken.

Fix error message about invalid surrogate pairs

Add CMake fetchcontent documentation and tests

I tested following strings with invalid surrogate pair and unpaired surrogate in files:

1. `"a\uD800\uD800x"`
2. `"a\uD800x"`

The error messge was: "... invalid string: surrogate U+DC00..U+DFFF must be followed by U+DC00..U+DFFF; ..."

I think it must be: "... invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; ..."

Co-Authored-By: Niels Lohmann <niels.lohmann@gmail.com>

Properly pass serialize_binary to dump function

Github issue:
https://github.com/nlohmann/json/issues/2073

nlohmann::json documents 2 way of depending on it using CMake
1) Copy-paste the project/source into your own project.
2) Install nlohman::json and then use find_package.

(1) pollutes your git repository, (2) requires everyone to install the
dependencies themselves.

Since 2018, CMake provide some kind of 'package manager' features using
[FetchContent](https://cmake.org/cmake/help/v3.17/module/FetchContent.html)
It gives the following:
~~~cmake
include(FetchContent)

FetchContent_Declare(json
  GIT_REPOSITORY https://github.com/nlohmann/json
  GIT_TAG v3.7.3)

FetchContent_GetProperties(json)
if(NOT json_POPULATED)
  FetchContent_Populate(json)
  add_subdirectory( ${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()
~~~

Then declares the dependency in the target using it:
~~~cmake
target_link_library(my_project PRIVATE nlohmann_json::nlohmann_json
~~~

This patch updates the documentation and provides tests.

Fix returning reference to local temporary object

updated wandbox link

Add missing testcase about NaN in unit-constructor1.cpp

Fix bug in diff function

Fix GCC compiler warnings

Fix Clang compiler warnings

Update doctest to 2.3.7

make CMake's version config file architecture-independent