Fix global loadstring lookup under strict mode #50

[harningt]

Description

This pull request resolves Issue #50 where importing/using the json module under strict mode (e.g. from strict.lua) fails.

The failure happens under Lua 5.2+ because loadstring or load performs a global lookup for the loadstring variable. Since loadstring is not declared or defined in these versions, accessing it directly triggers the global table's __index metamethod, causing a strict mode violation error:

variable 'loadstring' is not declared

Proposed Changes

• Safe loadstring Check: Replaced loadstring or load with rawget(_G or {}, "loadstring") or load in:
  • lua/json/encode/output.lua
  • lua/json/encode/output_utility.lua
  • tests/hook_require.lua
• Strict Mode Test Coverage: Implemented a filtered strict mode check in tests/hook_require.lua when the TEST_STRICT environment variable is active. The check is configured to only throw for undeclared global accesses originating from the library files (lua/json/*), bypassing third-party loader environments (like luarocks loader which references unpack).
• CI Integration: Configured the GitHub Actions testing matrix (.github/workflows/test.yml) to run the entire test suite (across all Lua versions and interpreters) with TEST_STRICT=1 enabled.

Closes #50

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.90%. Comparing base (65f7d76) to head (30bac5e).
⚠️ Report is 13 commits behind head on develop.
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #52      +/-   ##
===========================================
+ Coverage    95.69%   95.90%   +0.21%     
===========================================
  Files           18       18              
  Lines         1021     1026       +5     
===========================================
+ Hits           977      984       +7     
+ Misses          44       42       -2     

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