[wasm] emit producer section

Feature request

What is the expected behavior? Since https://github.com/WebAssembly/tool-conventions/pull/65, tooling is expected to emit the custom producer section.

Note that we should only add the Webpack entry and preserve the original producer.

What is motivation or use case for adding/changing the behavior? For future pipeline/tooling analysis.

How should this be implemented in your opinion? webassemblyjs

Are you willing to work on this yourself? yes