Split the Visual Studio 2019 AppVeyor job into two shards

The Visual Studio 2019 job (Release, x86, JSON_ImplicitConversions=OFF)
builds the tests for all standards and takes 49 to 57 minutes when it
passes, close to AppVeyor's 60-minute limit per job. In build 54917118
(#5867), build 54917312 (#5620), and build 54930878 (#5851) it was
stopped at the limit while still compiling. Split it with JSON_TestShard
like the Visual Studio 2017 jobs.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/external_ci/appveyor.yml

@@ -18,7 +18,8 @@ environment:
   matrix:
     # The Visual Studio 2017 jobs compile everything with /std:c++17, so they
     # only build the C++17 variant of each test, split into two jobs each to
-    # stay below AppVeyor's 60-minute limit per job.
+    # stay below AppVeyor's 60-minute limit per job. The Visual Studio 2019
+    # job builds the tests for all standards and is split for the same reason.
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       configuration: Debug
       platform: x86
@@ -50,7 +51,13 @@ environment:
       configuration: Release
       platform: x86
       CXX_FLAGS: "/W4 /WX"
-      CMAKE_OPTIONS: "-DJSON_ImplicitConversions=OFF"
+      CMAKE_OPTIONS: "-DJSON_ImplicitConversions=OFF -DJSON_TestShard=0/2"
+      GENERATOR: Visual Studio 16 2019
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      configuration: Release
+      platform: x86
+      CXX_FLAGS: "/W4 /WX"
+      CMAKE_OPTIONS: "-DJSON_ImplicitConversions=OFF -DJSON_TestShard=1/2"
       GENERATOR: Visual Studio 16 2019
 
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015