✨ System timezone on UNIX systems

[GiyoMoon]

This pull request adds support for system timezone on UNIX systems. It parses the TZif file under /etc/localtime to get the UTC offset at any given timestamp.

https://www.rfc-editor.org/rfc/rfc8536.html

• Basic implementation
• Correctly parse and use the TZ string from the footer in Version 3 TZif files see here
• Proper error handling
• Tests

[codecov-commenter]

Codecov Report

Merging #3 (773ba48) into main (985b2b2) will decrease coverage by 0.01%.
The diff coverage is 99.93%.

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@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
- Coverage   99.92%   99.91%   -0.01%     
==========================================
  Files          18       25       +7     
  Lines        3860     4911    +1051     
==========================================
+ Hits         3857     4907    +1050     
- Misses          3        4       +1     

Files	Coverage Δ
src/cron.rs	99.07% <100.00%> (-0.03%)	⬇️
src/date.rs	100.00% <100.00%> (ø)
src/datetime.rs	100.00% <100.00%> (ø)
src/local/cursor.rs	100.00% <100.00%> (ø)
src/local/data_block.rs	100.00% <100.00%> (ø)
src/local/errors.rs	100.00% <100.00%> (ø)
src/local/header.rs	100.00% <100.00%> (ø)
src/local/timezone.rs	100.00% <100.00%> (ø)
src/local/transition_rule.rs	100.00% <100.00%> (ø)
src/shared.rs	100.00% <ø> (ø)
... and 4 more