feat: add isCompressed() detection API with heuristics and tests

[zaenalcoders]

Summary

This PR introduces a new utility function isCompressed() for LZ-String, which provides a universal heuristic-based detection for strings or Uint8Array that are likely compressed by LZ-String. It covers all main compression outputs:

• compress() → UTF-16 characters

• compressToUTF16() → special leading header

• compressToBase64() → Base64-encoded

• compressToEncodedURIComponent() → URI-safe alphabet

• compressToUint8Array() → binary array

Motivation

Currently, there is no built-in way to determine if a string or buffer has been compressed by LZ-String without attempting decompression. This utility:

• Offers a lightweight, non-destructive check

• Supports all standard LZ-String compression formats

• Enhances usability for developers building pipelines or validating inputs

Implementation

• Added src/isCompressed/isCompressed.ts

• Added heuristic helper functions: looksLikeUTF16, looksLikeUTF16Special, looksLikeBase64, looksLikeURIEncoded

• Added comprehensive Vitest test suite in src/isCompressed/__test__/isCompressed.test.ts

• Includes JSDoc for clear documentation

• Fully backward-compatible, no changes to existing compression/decompression logic

Tests

All tests pass with Vitest:

npx vitest run src/isCompressed/__test__/isCompressed.test.ts

Covers:

• Positive detection for all compression methods

• Negative detection for raw strings, Base64/URI-like strings not compressed

• Edge cases: empty string, non-string inputs, Uint8Array inputs

Notes

• Non-breaking change

• Minimal footprint, fully optional for users