log-levels concept exercise needs a more fleshed-out stub

[glennj]

Two points

• this is an exercise about strings, so the student should not have to worry about the mechanics of functions
• working in the web editor, the student can't see the tests so they don't know what they should be called.

https://discord.com/channels/854117591135027261/1157359032760287302/1205523943021219850

[glennj]

The stub file could contain this:

namespace log_line {
    std::string message(const std::string& line) {
        // return the message
    }

    std::string log_level(const std::string& line) {
        // return the log level
    }

    std::string reformat(const std::string& line) {
        // return the reformatted message
    }
}

[vaeng]

I agree with you, the students can learn probably learn more from strings, when the functions are out of the way. At this point they do not know about const and references yet, so I would keep it very basic.

[vaeng]

I made a pr #806