zig: add JSValue assertion to cppFn

[nektro]

progress towards #15141
followup/similar to #15362

[robobun]

❌ @nektro, your commit dff4ae9 has 20 failures in #6977:

test/js/node/test/parallel/worker-nested-uncaught.test.js - segmentation fault on 🐧 3.20 x64

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/integration/next-pages/test/dev-server.test.ts - 1 failing on 🐧 3.20 x64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 aarch64

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64-baseline

test/v8/v8.test.ts - 22 failing on 🐧 3.20 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 22.04 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 22.04 x64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 aarch64

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64-baseline

test/js/node/test/parallel/fs-watch-recursive-linux-parallel-remove.test.js - 1 failing on 🐧 3.20 x64

test/js/node/test/parallel/timers-ordering.test.js - 1 failing on 🐧 22.04 x64-baseline

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 aarch64

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64-baseline

test/js/node/child_process/child_process.test.ts - 1 failing on 🐧 3.20 x64

test/js/bun/http/serve.test.ts - SIGTRAP on 🐧 3.20 aarch64

test/js/bun/http/serve.test.ts - SIGILL on 🐧 3.20 x64-baseline

test/js/bun/http/serve.test.ts - SIGILL on 🐧 3.20 x64

test/integration/next-pages/test/next-build.test.ts - 1 failing on 🐧 22.04 x64-baseline

test/js/deno/performance/performance.test.ts - 1 failing on 🐧 22.04 x64-baseline

test/bake/dev/esm.test.ts - 1 failing on 🐧 22.04 x64

test/js/third_party/jsonwebtoken/header-kid.test.js - internal assertion failure on 🪟 2019 x64-baseline

test/js/third_party/jsonwebtoken/header-kid.test.js - internal assertion failure on 🪟 2019 x64

test/js/node/test/parallel/http-date-header.test.js - internal assertion failure on 🪟 2019 x64-baseline

test/js/node/test/parallel/http-date-header.test.js - internal assertion failure on 🪟 2019 x64

test/js/third_party/jsonwebtoken/claim-private.test.js - internal assertion failure on 🪟 2019 x64-baseline

test/js/third_party/jsonwebtoken/claim-private.test.js - internal assertion failure on 🪟 2019 x64

test/js/bun/dns/resolve-dns.test.ts - 1 failing on 🪟 2019 x64-baseline

test/js/node/test/parallel/process-constants-noatime.test.js - internal assertion failure on 🪟 2019 x64-baseline

test/js/node/test/parallel/process-constants-noatime.test.js - internal assertion failure on 🪟 2019 x64

test/js/third_party/jsonwebtoken/claim-aud.test.js - internal assertion failure on 🪟 2019 x64

test/js/third_party/jsonwebtoken/claim-aud.test.js - internal assertion failure on 🪟 2019 x64-baseline

test/js/bun/test/expect.test.js - internal assertion failure on 🪟 2019 x64-baseline

test/js/bun/test/expect.test.js - internal assertion failure on 🪟 2019 x64

test/js/node/test/parallel/http2-compat-serverresponse-headers-send-date.test.js - internal assertion failure on 🪟 2019 x64

test/js/node/test/parallel/http2-compat-serverresponse-headers-send-date.test.js - internal assertion failure on 🪟 2019 x64-baseline

test/js/bun/ffi/ffi.test.js - internal assertion failure on 🪟 2019 x64-baseline

test/js/bun/ffi/ffi.test.js - internal assertion failure on 🪟 2019 x64

test/js/third_party/jsonwebtoken/async_sign.test.js - internal assertion failure on 🪟 2019 x64-baseline

test/js/third_party/jsonwebtoken/async_sign.test.js - internal assertion failure on 🪟 2019 x64

[Jarred-Sumner]

Fancy, but this adds many duplicate exception checks. Can we avoid duplicate exception checks?

[nektro]

per-irl discussion
.get uses a manual extern fn so this doesn't overlap there
callsite checks are 100% due for audit/removal/rewrite
previous pr adds check at different boundary so there's no overlap there