feat: check string length before adding newline

In literal strings, only the first newline will be trimmed.
```toml
str = '''
The first newline will be trimmed.'''
```
The previous code always adds this first-newline, but after this commit
it checks the length of the string and adds newline if the string is
sufficiently long.

toml/serializer.hpp

@@ -200,7 +200,11 @@ struct serializer
             if(std::find(s.str.cbegin(), s.str.cend(), '\n') != s.str.cend() ||
                std::find(s.str.cbegin(), s.str.cend(), '\'') != s.str.cend() )
             {
-                const std::string open("'''\n");
+                std::string open("'''");
+                if(this->width_ + 6 < s.str.size())
+                {
+                    open += '\n'; // the first newline is ignored by TOML spec
+                }
                 const std::string close("'''");
                 return open + s.str + close;
             }