fix: resolve webpack warning for optional date-fns-tz dependency

[martijnrusschen]

Summary

• Uses __non_webpack_require__ to load the optional date-fns-tz module
• This tells webpack to use the native Node.js require at runtime instead of trying to bundle/analyze the module
• Eliminates the "Critical dependency: the request of a dependency is an expression" warning
• Added: Handles ESM-only environments (like Vite) where require is not available

Background

When date-fns-tz is not installed (it's an optional peer dependency), webpack emits a warning because it can't statically analyze the dynamic require(variable) pattern. By using webpack's special __non_webpack_require__ global, we bypass webpack's bundling analysis while still falling back to regular require in non-webpack environments.

Vite Support

Vite and other ESM-only bundlers don't provide a require function. This PR adds a guard to check if require is available before attempting to use it, preventing runtime errors. However, due to the limitations of ESM, Vite users who want timezone support will need to configure their bundler - see #6204 for details.

Test plan

• TypeScript type checking passes
• All tests pass
• Build succeeds and includes the fix in dist output

Fixes #6181
Fixes #6204

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.29%. Comparing base (644f2f5) to head (f492f03).
⚠️ Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6182      +/-   ##
==========================================
- Coverage   99.31%   99.29%   -0.03%     
==========================================
  Files          30       30              
  Lines        3810     3826      +16     
  Branches     1639     1653      +14     
==========================================
+ Hits         3784     3799      +15     
- Misses         25       26       +1     
  Partials        1        1              

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