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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[PurkkaKoodari]

When require is not set, the code in f492f03 never reaches a path that would set dateFnsTz to something other than null. I don't think that's resolved by any dependency pre-bundling change in Vite.

Here's a relatively recent StackOverflow thread with related example code. It only talks about import, and I think you may need to use import in ESM... which also brings its own can of worms with it being async. Other options might be a "bring your own date-fns-tz" prop or a separate component that unconditionally pulls in date-fns-tz.

[PurkkaKoodari]

Hmm, in fact, presumably after #6155, I can't get timeZone to work even with Webpack and the hello-world application in the repo.

FWIW, I've made another PR #6227 which provides another way forward, but I think that might need at least a docs update if this is broken in Webpack as well.