Add proper error handling

[dfintha]

The current implementation simply returns std::nullopt on any kind of failure.

1. Track simple source location. Right now, a simple size_t holds the character index of each token.
2. Implement a simple result structure similar to std::result::Result or std::expected. (Can be omitted by using C++23.)
3. Implement error types for the separate steps: tokenizer, parser, optimizer, evaluator.
4. Use the above structure to propagate errors.
5. Provide proper error output, describing errors.