refactor: rename a function name and a var name

[cakebaker]

This PR renames the function tzname_to_offset to timezone_name_to_offset to be consistent with other function names in the file that all use timezone instead of tz. And it renames newtz to new_tz to be consistent with another variable in the same function named other_tz.

[yuankunzhang]

Nice one!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (7d0c3e4) to head (c87e103).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #214   +/-   ##
===========================
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