✨ implemented JSON Merge Patch (RFC 7396)

SQLite's json1 extension (https://www.sqlite.org/json1.html) supports JSON Merge Patch (https://tools.ietf.org/html/rfc7396). As the implementation is trivial and we already support JSON Patch, I think this could be a nice extension to the library.

test/Makefile

@@ -28,6 +28,7 @@ SOURCES = src/unit.cpp \
           src/unit-iterator_wrapper.cpp \
           src/unit-iterators1.cpp \
           src/unit-iterators2.cpp \
+          src/unit-merge_patch.cpp \
           src/unit-json_patch.cpp \
           src/unit-json_pointer.cpp \
           src/unit-meta.cpp \