[Feature Request] Support building for both UMD/ESM

When building wasm, I have to choose one between UMD and ESM for the generated JavaScript file, by either specifying the extension (.js vs .mjs) or using `-sEXPORT_ES6=1`. I want to build both of them but seems no way to do that.

Building it twice is not a working workaround because some number in the generated `ASM_CONSTS` table is different in 2 builds and the generated 2 JS files cannot use with one .wasm file.