build(bindgen): check for corresponding .zig file

[DonIsaac]

When generating bindings for foo.bind.ts, ensure a foo.zig file exists next to it. Builds will fail downstream if this is missing, this error just makes it surface sooner and in a more clear manner.

[robobun]

^{Updated 7:21 PM PT - Dec 27th, 2024}

❌ @DonIsaac, your commit 8e4d101 has 1 failures in #8611:

• test/js/bun/http/serve-body-leak.test.ts - timeout on 🪟 2019 x64-baseline
• test/js/bun/http/serve-body-leak.test.ts - timeout on 🪟 2019 x64

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🧪   try this PR locally:

bunx bun-pr 15896

[paperdave]

thank you for adding the additional assertion