wasm2js: Support elem.drop

[tlively]

In wasm2js, element segments are statically initialized into
FUNCTION_TABLE at module startup, and table.init is not supported,
so elem.drop has no observable effect and can be treated as a no-op.

[kripken]

Is there a reason to support this? In general we have not been adding new feature support to wasm2js.

[tlively]

#9201 makes the fuzzer emit elem.drop and starts hitting this WASM_UNREACHABLE. I guess we generally support running wasm2js when bulk-memory is enabled, so it's better to add this missing support than to disable wasm2js on bulk-memory and regress our coverage.

[kripken]

I see, thanks, sgtm as a simple fuzzer workaround.