build: npm run build (experimental) (#32823)

This PR implements much of the static assets rework ADR [1], including:

* `npm run build[-dev]`, and its subcommands,
* `npm run webpack[-dev]` and
* `npm run compile-sass[-dev]`.

This is backwards-compatible. `paver update_assets` should not be affected.
The new command warns that it is "experimental" for a few reasons:

* `npm run build` will fail in the webpack phase unless you first
run  `xmodule_assets`. This will be changed soon [2].

* We have tested the new build, but not quite so thoroughly that we'd
recommend it as the production default yet. Once the xmodule_assets
work lands, we'll share this on the forums so early adopters can try it
out.

* The commands lack some top-level documentation. Once they stabilize more,
we'll add a section to the README that explains how and when to use `npm run
build` and its subcommands and its env vars.

* `npm run watch` is not yet implemented.

References:
1. https://github.com/openedx/edx-platform/blob/master/docs/decisions/0017-reimplement-asset-processing.rst
2. https://github.com/openedx/edx-platform/pull/32685

Part of: https://github.com/openedx/edx-platform/issues/31604

webpack.dev.config.js

@@ -21,7 +21,7 @@ module.exports = _.values(Merge.smart(commonConfig, {
             }),
             new webpack.DefinePlugin({
                 'process.env.NODE_ENV': JSON.stringify('development'),
-                'process.env.JS_ENV_EXTRA_CONFIG': process.env.JS_ENV_EXTRA_CONFIG
+                'process.env.JS_ENV_EXTRA_CONFIG': process.env.JS_ENV_EXTRA_CONFIG || "{}"
             })
         ],
         module: {