Merge pull request #937 from xiemaisi/js/e4x

JavaScript: Add support for E4X.

Author: Esben Sparre Andreasen

change-notes/1.20/extractor-javascript.md

@@ -19,6 +19,7 @@
 ## Changes to code extraction
 
 * Extraction of JavaScript files (but not TypeScript files) on LGTM is now parallelized. By default, the extractor uses as many threads as there are processors, but this can be overridden by setting the `LGTM_INDEX_THREADS` environment variable. In particular, setting `LGTM_INDEX_THREADS` to 1 disables parallel extraction.
+* The extractor now offers experimental support for [E4X](https://developer.mozilla.org/en-US/docs/Archive/Web/E4X), a legacy language extension developed by Mozilla.
 * The extractor now supports additional [Flow](https://flow.org/) syntax.
 * The extractor now supports [Nullish Coalescing](https://github.com/tc39/proposal-nullish-coalescing) expressions.
 * The extractor now supports [TypeScript 3.2](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-2.html).