Codechange: vendor the nlohmann-json library (#11290)

(cherry picked from commit ba67f39db6)

# Conflicts:
#	.github/workflows/ci-build.yml
#	CMakeLists.txt
#	COMPILING.md
#	src/os/macosx/survey_osx.cpp
#	src/os/windows/survey_win.cpp
#	src/script/api/script_admin.cpp
#	src/script/api/script_event_types.cpp
#	src/tests/test_script_admin.cpp

COMPILING.md

@@ -4,7 +4,6 @@
 
 OpenTTD makes use of the following external libraries:
 
-- (required) nlohmann-json: JSON handling
 - (encouraged) zlib: (de)compressing of old (0.3.0-1.0.5) savegames, content downloads,
    heightmaps
 - (encouraged) liblzma: (de)compressing of savegames (1.1.0 and later)
@@ -55,14 +54,13 @@ the `static` versions, and OpenTTD currently needs the following dependencies:
 - libzstd
 - libpng
 - lzo
-- nlohmann-json
 - zlib
 
 To install both the x64 (64bit) and x86 (32bit) variants (though only one is necessary), you can use:
 
 ```ps
-.\vcpkg install liblzma:x64-windows-static zstd:x64-windows-static libpng:x64-windows-static lzo:x64-windows-static nlohmann-json:x64-windows-static zlib:x64-windows-static
-.\vcpkg install liblzma:x86-windows-static zstd:x86-windows-static libpng:x86-windows-static lzo:x86-windows-static nlohmann-json:x86-windows-static zlib:x86-windows-static
+.\vcpkg install liblzma:x64-windows-static zstd:x64-windows-static libpng:x64-windows-static lzo:x64-windows-static zlib:x64-windows-static
+.\vcpkg install liblzma:x86-windows-static zstd:x86-windows-static libpng:x86-windows-static lzo:x86-windows-static zlib:x86-windows-static
 ```
 
 You can open the folder (as a CMake project). CMake will be detected, and you can compile from there.