MSVC: make #4526 pass (use #if for optional) - fixes #3859; re-amalgamate

[chelseadzdr]

Changes made

• pulled pr optional: use #if instead of #ifndef, fixes #3859 #4526 and amalgamated
• merged into develop and fixed conflicts, kept most recent version of from_json.hpp (removed extra JSON_USE_IMPLICIT_CONVERSIONS that broke C++11 builds).
• Regenerated single-header files via the amalgamation target.

Existing issue is referenced:

• .value() with optional default value fails to compile #3859

How I tested

• Windows: VS 2019, x64 toolset, CMake
  • -DJSON_BuildTests=ON
  • Built and ran tests (Debug/Release).

Amalgamation

• The source code is amalgamated by running py -3 tools\amalgamate\amalgamate.py.

[gregmarr]

I don't understand what's happening with this PR. It is not showing the change in the original file, but it is showing it in the amalgamated file, and there's no complaint about that from the checks. Also strange, there are only 5 checks.

[chelseadzdr]

@gregmarr i think most workflows are awaiting approval because it is my first contribution, and the changes in the source files has been made in previous commits, the latest commits just amalgamated the headers, that's why at first it may look like only the amalgamated files are modified

[github-actions]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @chelseadzdr
Please read and follow the Contribution Guidelines.

[coveralls]

coverage: 99.191%. remained the same
when pulling 4c55fcb on chelseadzdr:fix-msvc-optional
into 4f8ee1d on nlohmann:develop.