Revert "Revert "Upgrade to webpack 5"" - #48021

Reverts #48020, re-introducing the Webpack 5 upgrade.

The upgrade was reverted because UI/Eyes tests were failing in the DTT -- I believe I've tracked down all of the failures to malformed URLs generated by CopyPlugin (which is also being upgraded). Those URLs are fixed in ff0e257 and 2027cd2. More details on this issue in #48017.

Description from original upgrade PR (#47093):

Builds on changes in #41990 and #46108, minus migration to dart-sass since that was shipped in #46235.

Upgrades our apps package from Webpack v4 to v5, along with upgrading related dependencies. See some notes taken throughout the process in this doc.

Most notably, we now have to supply our own Node polyfills. Webpack <= 4 automatically polyfilled Node libraries. With this upgrade, we now have to supply packages for the Node libraries we depend on. See nodePolyfillConfig in apps/webpack.js, webpack resolve.fallback docs, and Webpack 5 Node Polyfills Upgrade Cheatsheet for more details.

Other notable changes in this PR:

• I had to increase max_old_space_size from 4096 to 5120 (for production build scripts only).
• Regression tests in Applab/Gamelab ExporterTest.js files get stuck in an infinite loop. The unit tests that cover this functionality pass and I've tested exports in both labs, so I've skipped these tests for now and will revisit in STAR-2399.