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-cbor.cpp

@@ -1287,7 +1287,7 @@ TEST_CASE("CBOR")
         }
     }
 
-    SECTION("additonal deserialization")
+    SECTION("additional deserialization")
     {
         SECTION("0x7b (string)")
         {

test/src/unit-class_parser.cpp

@@ -210,7 +210,7 @@ json parser_helper(const std::string& s)
     json j;
     json::parser(nlohmann::detail::input_adapter(s)).parse(true, j);
 
-    // if this line was reached, no exception ocurred
+    // if this line was reached, no exception occurred
     // -> check if result is the same without exceptions
     json j_nothrow;
     CHECK_NOTHROW(json::parser(nlohmann::detail::input_adapter(s), nullptr, false).parse(true, j_nothrow));

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",

test/src/unit-regression.cpp

@@ -164,7 +164,7 @@ TEST_CASE("regression tests")
 {
     SECTION("issue #60 - Double quotation mark is not parsed correctly")
     {
-        SECTION("escape_dobulequote")
+        SECTION("escape_doublequote")
         {
             auto s = "[\"\\\"foo\\\"\"]";
             json j = json::parse(s);

test/src/unit-udt.cpp

@@ -238,7 +238,7 @@ void from_json(const nlohmann::json& j, contact_book& cb)
 TEST_CASE("basic usage" * doctest::test_suite("udt"))
 {
 
-    // a bit narcissic maybe :) ?
+    // a bit narcissistic maybe :) ?
     const udt::age a
     {
         23

test/src/unit-unicode.cpp

@@ -1152,7 +1152,7 @@ TEST_CASE("Unicode" * doctest::skip())
         json j;
         CHECK_NOTHROW(f >> j);
 
-        // the array has 1112064 + 1 elemnts (a terminating "null" value)
+        // the array has 1112064 + 1 elements (a terminating "null" value)
         // Note: 1112064 = 0x1FFFFF code points - 2048 invalid values between
         // 0xD800 and 0xDFFF.
         CHECK(j.size() == 1112065);