### Description
* follows the packaging approach according to the design document
* adds `ENABLE_TRAINING` boolean flag to `BUILD_DEFS`
* modifies `package.json` to include training submodule
* modifies build script to handle, validate, and minimize training WASM
artifacts
* adds the binding for the new backend with training enabled & the new
training artifacts
* adds training backend
* edits `index.ts` to use training backend depending on `BUILD_DEFS`
* edits `wasm-factory.ts` to use the training artifacts if necessary
### Motivation and Context
* we are in the process of adding web bindings to enable training.
* Adding the "glue" to allow onnxruntime-web to use the training WASM
artifacts is required for this work.
* Since BUILD_DEFS is defined and used at build time, I thought that it
made sense to bundle the changes to building in the same PR.
#### Related work
* #16521 allowed for training artifacts to be built
* #17333 must be merged in before this one
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Co-authored-by: Yulong Wang <7679871+fs-eire@users.noreply.github.com>
