fix: Pandoc-like attributes are too restrictive

[Omikhleia]

Closes #62

Carefully reading Pandoc's code, I hope I got it right now:

1. attributes key=value pairs
   • The key must start with a letter, followed by alphanumeric characters or _-:. (rather than just _-)
   • The value may be double-quoted or single-quoted, and has less restrictions (than initially implemented)
2. class .class identifiers
   • Same rule as for attribute keys
3. hash #id identifiers
   • The identifier syntax was relaxed in Markdown writer produces numeric identifiers pandoc#7920 and they are thus different (allowed not to start with a letter)
4. raw =raw attributes
   • They have yet another syntax, consisting in alphanums and _- only

[Witiko]

Independently implemented 1 and 2 in Witiko#297. 3 and 4 are a good catch and I will be porting them to witiko/markdown in Witiko#304, possibly catching some further bugs in the process. In the meantime: can you please extend the unit tests for fenced code attributes and raw attributes with some examples to prevent future regressions?