build: configure webpack to handle UMD emits

> The vscode-json-languageservice module includes ESM and UMD emits, but sets the UMD output as the main one. Our webpack config needs extra configuration to ignore UMD distributions to avoid issues like this

- reference:
- https://dev.to/iggredible/what-the-heck-are-cjs-amd-umd-and-esm-ikm
- https://weblogs.asp.net/dixin/understanding-all-javascript-module-formats-and-tools
- #2141

Start by locating the webpack configuration and reviewing issue #2141 alongside the linked module-format references. Confirm the current build problem, then verify that UMD distributions are ignored and the webpack build completes without the reported issues.

Written by the indexing model from the issue text.