This commit introduces major improvements across the library.

- Added a thread-safe singleton for core data,
  making `Uroman::new()` near-zero cost after the first call.
  The library is now fully thread-safe.

- Fixed an out-of-bounds panic related to a Tibetan character.

- BREAKING: Renamed `RomanizationOutput::to_output_string()` to
  `to_string()` for better ergonomics and consistency with Rust conventions.