Change plugin-wasm to include outputVarIds in the generated module

[chrispcampbell]

Currently we have a roundabout way of initializing a WasmModel, where you have to pass the array of OutputVarId to initWasmModelAndBuffers. This adds to complexity in plugin-worker, where it has some special code to inject the outputVarIds into the generated worker.

If we instead change plugin-wasm to include the outputVarIds inside the generated wasm-model.js file (created by Emscripten), we can simplify the process of initializing a WasmModel.

Emscripten has a --pre-js argument that allows for injecting arbitrary code into the generated module, which we can use for this purpose.

This will be a simple change to plugin-wasm only for now; the generated module will continue to be compatible with the existing initialization process, but doing this change first will allow for further simplification in an upcoming PR.

[chrispcampbell]

With this change, plugin-wasm now generates a processed_outputs.js file into sde-prep/build that looks like this:

Module["outputVarIds"] = ["_y", "_z"];

Using emcc --pre-js build/processed-outputs.js will cause that file to be folded into the generated wasm-model.js file: