js: set emcc options when generating for the npm package

Author: jgriffiths

Diff for: src/wasm_package/build.sh

@@ -2,7 +2,7 @@
 set -xeo pipefail
 
 # Build WASM (Note Elements is always enabled)
-(cd ../.. && ./tools/build_wasm.sh)
+(cd ../.. && EMCC_OPTIONS="-s EXPORT_ES6=1 -s MODULARIZE=1" ./tools/build_wasm.sh)
 mkdir -p libwally_wasm && cp ../../dist/wallycore.{js,wasm} libwally_wasm/
 touch libwally_wasm/index # necessary for webpack to work (fixes "Can't resolve './' in 'wasm_package/libwally_wasm'")