Improving V8's error thrown when WebAssembly.instantiate(...) is used with a buffer

[KianNH]

Some libraries will output WebAssembly as a byte array in a JavaScript file, and attempt to instantiate from that. Since this array could also be constructed dynamically or from external sources, it is classed as code generation and disallowed in the same way that eval and new Function are.

The error thrown from v8/src/objects/contexts.cc#Context::ErrorMessageForWasmCodeGeneration() is Wasm code generation disallowed by embedder.

This sometimes leads to confusion by users or developers of WASM libraries:

Some libraries, like xxhash-wasm, now have a specific workers export that uses the module rather than instantiating from a buffer:

https://github.com/jungomi/xxhash-wasm/blob/5923f26411ed763044bed17a1fec33fee74e47a0/src/index.js#L8

https://github.com/jungomi/xxhash-wasm/blob/5923f26411ed763044bed17a1fec33fee74e47a0/src/index.workerd.js#L3-L4

cc @jasnell