Add BigInt fallback for Int64/UInt64 div/mod on JS target

[Copilot]

The JS target used WebAssembly for 64-bit integer division and modulo operations, which fails in Cloudflare Workers where WebAssembly is restricted.

Changes

• Fallback detection: Added lazy initialization of WebAssembly helper with cached result to avoid repeated initialization attempts
• BigInt implementations: Added JavaScript BigInt-based div/mod operations for both signed and unsigned 64-bit integers
• Automatic fallback: Modified div/mod operations to use WebAssembly when available, falling back to BigInt when not

Implementation

The fallback is transparent - existing code continues to work without changes:

// Works in both standard JS and Cloudflare Workers
let result = 9223372036854775807L / 42L
let remainder = 18446744073709551615UL % 100UL

WebAssembly is preferred when available for performance. BigInt provides full correctness when WebAssembly is unavailable.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Make cloudflare worker our tier 1 js target</issue_title>
<issue_description>We should setup a github action for testing

ref: https://developers.cloudflare.com/workers/testing/miniflare/

TODO:

• Add fallback implementation (BigInt based) for div/mod of (U)Int64.</issue_description>

<agent_instructions>Add fallback implementation (BigInt based) for div/mod of (U)Int64.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Make cloudflare worker our tier 1 js target #2984

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[coveralls]

Pull Request Test Coverage Report for Build 1975

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.37%

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Totals
Change from base Build 1968:	0.0%
Covered Lines:	10185
Relevant Lines:	11147

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💛 - Coveralls