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

package.json

@@ -3,7 +3,13 @@
   "version": "0.1.0",
   "repository": "https://github.com/openedx/edx-platform",
   "scripts": {
-    "postinstall": "scripts/copy-node-modules.sh"
+    "postinstall": "scripts/copy-node-modules.sh",
+    "build": "echo 'WARNING: `npm run build` in edx-platform is experimental. Use at your own risk.' && npm run webpack && npm run compile-sass",
+    "build-dev": "echo 'WARNING: `npm run build-dev` in edx-platform is experimental. Use at your own risk.' && npm run webpack-dev && npm run compile-sass-dev",
+    "webpack": "NODE_ENV=${NODE_ENV:-production} \"$(npm bin)/webpack\" --config=${WEBPACK_CONFIG_PATH:-webpack.prod.config.js}",
+    "webpack-dev": "NODE_ENV=development \"$(npm bin)/webpack\" --config=webpack.dev.config.js",
+    "compile-sass": "scripts/compile_sass.py --env=${NODE_ENV:-production}",
+    "compile-sass-dev": "scripts/compile_sass.py --env=development"
   },
   "dependencies": {
     "@babel/core": "7.19.0",