refactor: simplify codebase by extracting helpers and reducing duplication

- Extract pad2/pad3 formatting helpers in birl.gleam, replacing dozens of
  repeated int.to_string |> string.pad_start patterns
- Extract validate_timezone helper to deduplicate timezone validation
- Reuse existing to_nanoseconds helper in compare, scale_up, scale_down
- Consolidate new/accurate_new and decompose/accurate_decompose via shared
  parameterized helpers in duration.gleam
- Replace single-arm case destructuring with let destructuring in interval.gleam
- Simplify now_with_timezone and set_timezone using result.try
- Simplify is_invalid_date/is_invalid_time with direct boolean expressions
- Remove redundant int.absolute_value call in legible_difference

Author: tylerbutler