feat: add Base85 (ASCII85) cipher implementation

[AliAlimohammadi]

Description

This PR adds an implementation of the Base85 (ASCII85) encoding and decoding cipher to the ciphers module.

Base85, also known as ASCII85, is a binary-to-text encoding scheme developed by Adobe Systems. It encodes 4 bytes of binary data into 5 ASCII characters, using characters in the range 33-117 ('!' to 'u'). This encoding is more efficient than Base64, producing approximately 25% less overhead.

Changes Made

• ✅ Added base85.rs with base85_encode and base85_decode functions
• ✅ Updated mod.rs to include the new base85 module and exports
• ✅ Updated DIRECTORY.md to list the new cipher (alphabetically ordered)
• ✅ Included comprehensive documentation with examples
• ✅ Added unit tests including edge cases and roundtrip tests

Implementation Details

• Encoding: Converts binary data to Base85 by processing 32-bit chunks
• Decoding: Converts Base85 text back to binary data
• Edge Cases: Properly handles empty input and padding requirements
• Tests: Includes tests for empty strings, standard cases, and encode/decode roundtrips

Testing

All tests pass:

cargo test --lib ciphers::base85

Example usage:

use the_algorithms_rust::ciphers::{base85_encode, base85_decode};

let encoded = base85_encode(b"Hello");
let decoded = base85_decode(&encoded);
assert_eq!(decoded, b"Hello");

References

• Wikipedia: ASCII85

Checklist

• Code follows the repository's style guidelines
• Documentation includes examples and references
• All tests pass
• mod.rs updated with new module
• DIRECTORY.md updated with new cipher
• No warnings from cargo clippy
• Code is formatted with cargo fmt

[AliAlimohammadi]

@siriak, this is ready to be merged.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 99.07407% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.01%. Comparing base (43c7049) to head (6caca6b).

Files with missing lines	Patch %	Lines
src/ciphers/base85.rs	99.07%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1018      +/-   ##
==========================================
+ Coverage   95.99%   96.01%   +0.01%     
==========================================
  Files         382      383       +1     
  Lines       28744    28852     +108     
==========================================
+ Hits        27594    27702     +108     
  Misses       1150     1150              

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[siriak]

Looks good, thanks!