[logdecoder] Improve log decoding performance by 9x

* DON'T SEEKG BACKWARDS (LOL)! This was done in an attempt to "cleanly" replace the last comma of a CSV row with a newline, but turns out, it absolutely destroys performance.
* Avoid using std::ifstream's get() or peek() in a loop, use getline() and eof() to reduce sentry object creation overhead (as seen from profiling) and use ignore() where we don't care about the content for bonus readibility points.
* Use std::format for faster output formatting (especially floating point)
* Replace template specialization with constexpr-if because it's cooler and we already have C++20 so why not