Add JLD2 file format support

[Copilot]

Issue #12 requested JLD2 support for better handling of arbitrary Julia types (e.g., BigInt) that BSON struggles with.

Changes

• File format detection: Extension-based dispatch (.bson → BSON.jl, .jld2 → JLD2.jl)
• Metadata handling: BSON uses Symbol keys, JLD2 uses String keys
• Function updates: Modified cache() to detect extension and call appropriate save/load functions
• Validation: Early extension checking with clear error messages for unsupported formats
• Tests: Added 21 tests covering function form, macro form, module loading, and error handling
• Documentation: Updated docstrings and README with file format examples

Usage

# BSON format (existing)
cache("results.bson") do
    x = compute_expensive_result()
end

# JLD2 format (new)
cache("results.jld2") do
    x = big"123456789012345678901234567890"  # BigInt works better with JLD2
end

Both formats preserve all existing functionality. Macro form @cache works with both extensions automatically.

Warning

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Original prompt

Please open a new PR to address #12. The PR should involve minimal changes to the code, only modifying how the saving/loading is done. Make sure to add some corresponding tests to exercise the new code.

---

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5b86c69) to head (80e5968).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #26   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           40        57   +17     
=========================================
+ Hits            40        57   +17     

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[dahong67]

Revised and looking good. Ready to merge!