fix: patch __dirname for ESM environments

[azohra]

Hey! Ran into this issue deploying to Cloudflare Workers and dug into the root cause.

The problem

The generated libh3.js uses __dirname for Node detection:

scriptDirectory=__dirname+"/"

This breaks in ESM environments where __dirname doesn't exist — Cloudflare Workers, Node ESM, Deno, Bun, etc.

The fix

Added a sed replacement in the build script (same approach as the existing document.currentScript patch from #117 and #163):

scriptDirectory=typeof __dirname!=="undefined"?__dirname+"/":""

Why it's safe

h3-js embeds memory init as a base64 data URI, so scriptDirectory falling back to "" works fine — it's never actually used to resolve file paths in bundled deployments.

Testing

• Verified the sed pattern matches the generated out/libh3.js
• Tested in Cloudflare Workers — works
• Backwards compatible — Node CJS still gets __dirname as before

Fixes #216

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[azohra]

One note on approach — another option would be updating to Emscripten 3.x+ and adding -sUSE_ES6_IMPORT_META=1, which generates proper import.meta.url code instead of __dirname.

Went with the sed patch here since:

• It's a minimal change that follows the existing pattern in the build script
• Avoids the risk of updating the Emscripten toolchain (SDK 1.38.43 → 3.x is a big jump)
• Can always do the Emscripten upgrade separately if desired

Happy to take either direction if maintainers prefer the toolchain update instead!