Fix some spelling errors - mostly in comments & documentation.

I did not touch the Changelog file and any third party stuff.

additonal -> additional (1)
apppend -> append (2)
constuctor -> constructor (2)
contect -> context (2)
dobulequote -> doublequote (1)
elemnts -> elements (1)
exakt -> exact (2)
exluded -> exclude (1)
explicitely -> explicitly (2)
narcissic -> narcissistic (1)
ocurred -> occurred (1)
occuring -> occurring (2)
preceeds -> preceded (1)
ot -> to (2)
wehther -> whether (2)

test/src/unit-msgpack.cpp

@@ -1359,7 +1359,7 @@ TEST_CASE("MessagePack roundtrips" * doctest::skip())
 {
     SECTION("input from msgpack-python")
     {
-        // most of these are exluded due to differences in key order (not a real problem)
+        // most of these are excluded due to differences in key order (not a real problem)
         auto exclude_packed = std::set<std::string>
         {
             "test/data/json.org/1.json",