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-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);