fix: restrict length of underline by size of line

in some cases, `region` contains several lines and `region::size`
returns the whole size that is a sum of lengthes of all the lines.
To avoid too long underlines, restrict the length of underline by
the length of the line that is shown in the message.

toml/region.hpp

@@ -343,7 +343,8 @@ inline std::string format_underline(const std::string& message,
         {
             // invalid
             // ~~~~~~~
-            retval << make_string(reg->size(), '~');
+            const auto underline_len = std::min(reg->size(), reg->line().size());
+            retval << make_string(underline_len, '~');
         }
 
         retval << ' ';