Use internal result as regex return type

[jackzhxng]

🧱 Stack:

• Use common base class private functions for TikToken #45
• Add regex interface with re2 and std::regex implementations #48
• Decouple tokenizers from Re2 and use IRegex interface #49
• Add pcre2 as re2 fallback #50
• Use internal result as regex return type #51

[shoumikhin]

No ET code throws exceptions afaik, will we guard it from tokenizers now?